Sauron is not stupid! But he--like all of us--is limited by his picture of what the world is like:
[H]e is very wise, and weighs all things to a nicety in the scales of his malice. But the only measure that he knows is desire, desire for power; and so he judges all hearts. Into his heart the thought will not enter that any will refuse it, that having the Ring we may seek to destroy it. If we seek this, we shall put him out of reckoning.'
Like anyone with more problems than resources to deal with them, planning is a matter of cost/benefit.
Sauron judges (for good, though incorrect, reason) that the chance of someone trying to destroy the Ring is negligible.
The Enemy, of course, has long known that the Ring is abroad, and that it is borne by a hobbit. He knows now the number of our Company that set out from Rivendell, and the kind of each of us. But he does not yet perceive our purpose clearly. He supposes that we were all going to Minas Tirith; for that is what he would himself have done in our place. And according to his wisdom it would have been a heavy stroke against his power. Indeed he is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place. That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind. That we should try to destroy the Ring itself has not yet entered into his darkest dream.
So what should he do to deal with this highly unlikely case? Station guards all around Mt. Doom? No. This is very expensive (not only do you spend the guards, but you also need to supply them with food and water in a foodless and waterless wilderness). Big cost, low return.
But he knows that the Ring is at its most powerful at Mt. Doom and probably gains in power as you approach the mountain. It's a long shot, but maybe, just maybe, when Aragorn (or Boromir or whoever) takes the Ring for himself, A. or B. or W. will head off to Mt. Doom to launch his challenge of Sauron. Still too much of a long shot to warrant stationing an army around Mt. Doom, but it is worthwhile to alert the border guards -- which he needs anyway -- with orders to properly handle any Ringbearers they might capture. That's very low cost and is a sensible precaution against a low-probability event.
If Sauron had suspected that there was any chance at all that the Ring might be destroyed -- literally the most catastrophic possible event from his point of view -- then that chance would be enough to justify guards all around Mt. Doom.
Sauron is a rationalist, a techie, a hard-headed believer that good planning and self-interest is the best way forward--and he thinks everyone else is that way too and that he understands them. Again, I emphasize, he is not stupid and "all things [are weighed] to a nicety in the scales of his malice."
So, no guards ⇒ no suspicion.