I agree that Voldemort's arrogance likely played a major role in his presuming no one would discover where his Horcruxes were hidden. That he purposefully made six Horcruxes -- and, accidentally and unknowingly, a seventh with Harry (plus the residual piece of soul that resided in Voldemort himself -- the remnants of his original soul) -- likely rendered him secure in the belief that he was invincible.
However, in his arrogance, he couldn't keep from bragging about his achievements, so there were individuals who Voldemort clued in on the possibility of a Horcrux or Horcruxes existing. In speaking with his Death Eaters in Goblet of Fire, Voldemort obliquely hints at the Horcruxes and suggests that he had spoken of them previously:
"And then I ask myself, but how could they have believed I would not rise again? They, who knew the steps I took, long ago, to guard myself against mortal death? They who had seen proofs of the immensity of my power in the times when I was mightier than any wizard living?" (Goblet of Fire -- Chapter 33)
Through his superiority complex and belief that he was invincible, Voldemort dropped enough clues regarding the Horcruxes, that Regulus Black was able to work out what Voldemort was doing:
To the Dark Lord: I know I will be dead long before you read this but I want you to know that it was I who discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I can. I face death in the hope that when you meet your match, you will be mortal once more. R.A.B. (Half-Blood Prince -- Chapter 28)
Voldemort was complacent regarding youth and inexperience; it would have never occurred to him that such a young wizard would have been able to figure out the complexities of the locket Horcrux cave; however, Regulus Black did, and was able to successfully switch the real Horcrux with the fake locket with the assistance of Kreacher the House-elf. Furthermore, Voldemort failed to keep in mind that there is different kinds of magic than human wizarding magic; House-elves' magic is different than human magic, which ended up being a vulnerability to Voldemort that he never considered and therefore never guarded against. While Voldemort believed himself the most powerful wizard of all time, he, as Dumbledore noted throughout the series, remained woefully ignorant.
In his attempt to kill Harry, the rebounding curse should have killed Voldemort; however, it failed to do so, and he became even more smug:
"What I was, even I do not know. . . I, who have gone further than anybody along the path that leads to immortality. You know my goal -- to conquer death. And now, I was tested, and it appeared that one or more of my experiments had worked . . . for I had not been killed." (Goblet of Fire -- Chapter 33)
Interestingly, as a quick side note, this seems to confirm that a Horcrux is reusable. If "one or more of [Voldemort's] experiments had worked" -- meaning a Horcrux or Horcruxes -- kept Voldemort from being killed by his rebounding Avada Kedavra curse, the Horcruxes themselves were not negatively affected by mustering the power to prevent Voldemort's death. This likely further exacerbated Voldemort's complacency regarding the Horcruxes.
There is a lot of information regarding the Horcruxes in Half-Blood Prince in particular.
Regarding the diary Horcrux, Dumbledore notes that the diary was both a safeguard and a weapon. Voldemort's arrogance over making it known that he, Tom Riddle, was the Heir of Slytherin, and to further his work from the Chamber of Secrets via basilisk attacks. Dumbledore says:
"[. . . ] if he intended the diary to be passed to, or planted on, some future Hogwarts student, he was being remarkably blase about that precious fragment of his soul concealed within it. The point of a Horcrux is, as Professor Slughorn explained, to keep part of the self hidden and safe, not to fling it into somebody else's path and run the risk that they may destroy it -- as indeed happened." (Half-Blood Prince -- Chapter 23)
Voldemort protected his Horcruxes through powerful magical enchantments; however, he failed to anticipate that someone (Dumbledore) would begin collecting as much information on Voldemort as possible, allowing Dumbledore to piece together Voldemort's plan and, based on what he learns of and knows about Tom Riddle, be able to deduce where Voldemort had hidden the Horcruxes. Magic always leaves behind traces (Half-Blood Prince -- Chapter 26), so no spell or enchantment Voldemort could have used would have been completely undetectable. Regarding the ring Horcrux:
"He hid it, protected by many powerful enchantments, in the shack where his ancestors had once lived (Morphin [Gaunt] having been carted off to Azkaban, of course), never guessing that I might one day take the trouble to visit the ruin, or that I might be keeping an eye out for traces of magical concealment." (Half-Blood Prince -- Chapter 23)
As well, Voldemort's "pride, his belief in his own superiority, his determination to carve for himself a startling place in magical history," lead Voldemort to select objects with significant meaning within which to encase his Horcruxes, whether personal (the diary; the ring; Slytherin's locket; Nagini) or historically important (the cup; the diadem, and also the locket). It did not occur to Voldemort that someone like Dumbledore would be able to profile his personality accurately enough to deduce what kind of objects Voldemort had used to encase his Horcruxes. He felt he had hidden them carefully enough.
Voldemort may have believed that he would somehow know (hence be able to monitor) if one or more of his Horcruxes were destroyed; however, it's clear that this was not the case, as he did not know the diary Horcrux had been destroyed until he forced the truth out from Lucius Malfoy. With Malfoy, after giving Malfoy the diary, it did not occur to Voldemort that Malfoy would dare to do anything with the diary without Voldemort's express order. Yep, arrogance. Dumbledore says, regarding whether or not Voldemort can feel when a Horcrux is destroyed:
"I believe not. I believe that Voldemort is now so immersed in evil, and these crucial parts of himself have been detached for so long, he does not feel as we do. Perhaps at the point of death he might be aware of his loss . . ." (Half-Blood Prince -- Chapter 23)
Voldemort likely did not believe that there would ever be a wizard powerful enough to kill him, therefore lessening the urgency to guard or monitor his Horcruxes. Dumbledore says:
"It will take uncommon skill and power to kill a wizard like Voldemort, even without his Horcruxes." (Half-Blood Prince -- Chapter 23) Yet it was Voldemort himself who elevated Harry as his equal, by his acceptance of Sibyll Trelawney's prophecy and his marking of Harry as the boy in question who could vanquish the Dark Lord.
Voldemort believed that there was nothing worse than physical injury or death; he assumed others would hold the same belief, so when he made the spilling of blood necessary to open the sea cave with the locket Horcrux inside, it did not occur to him that others might come through who did not fear injury or death. Again, in Voldemort's estimation, no need to monitor the cave.
Regarding the diadem Horcrux, Voldemort was unable to procure a teaching position at Hogwarts, when Dumbledore failed to hire him on as the Defense Against the Dark Arts instructor, which denied him access to Hogwarts where the diadem was hidden in the Room of Requirement. Voldemort believed that only he had plumbed the depths of secrets contained at Hogwarts (the Chamber of Secrets, the Room of Requirement), it did not occur to him to have to guard the diadem against others. He believed that no one else could be smart enough to discover the Room of Requirement or the Chamber of Secrets.
In a nutshell, Voldemort was ultimately sloppy with his Horcruxes.
