My 7 year old has mystery testicles (i know as if life was not exciting enough). His scrotum largely seems empty but one usually makes an appearance. I asked about the other and he says it comes down only if it is very very warm & even then he wasn't overly sure.
Both were there and 'signed off' at birth but most of the time his scrotum seems empty (I do not spend every waking hour looking at it, he is a child however with no nudity issues).
Problem is that if you try to check for the spare one he finds it hilarious and everything head upwards.