What Nathan says is exactly what I think, even though I don't have nearly his experience. When I first started having colts gelded, I would check and check and wasn't totally sure I was feeling "them". They can hold them up so tight that it takes a lot of practice to tell that they're actually there by touch.
H and I have had lots of horses gelded over the years, several I of them it was only a ghost of a feel and wondering if for sure they were there (and they were -- but not for long as the vet did his thing!). The most recent one, one was visibly descended and the other was down but being held to the body and I was not completely sure I felt it then, but I knew I had in the fall (so wasn't worried). I was surprised, though, to see how big it actually was (maybe 2" long!). Even that size, he held it so snug I was not able to feel it prior to gelding but knew I had done so last fall.
Once though they are really down (including if they are down at birth) I do NOT think they can / do pass back through the ring into the body cavity. I think they are merely being held up tight to the body, and that most individuals are unable to feel them.
My vet has a reputation of being able to geld "the normal way" miniature horses that were called cryptorchid. I really wonder if it's just what I'm saying here (not that he's not awesome -- I adore him as a vet and a person). That people (even some vets?) call some of these minis cryptorchid when they really aren't.