Nobody knew about his Horcruxes (even Slughorn wasn't 100% sure). But still, there was a small probability that someone could discover his secret. So, he hid his Horcruxes very well. But, there was always a way to get to those Horcruxes.
Why didn't he hide his Horcruxes in a place which was out reach of anyone? Some examples: His own stomach, Center of Sun/Earth/Black Hole etc. He could also had simply made Voyager-1 a Horcrux..