I'm no expert, but I think it may be the undescended testicle, or maybe an inguinal hernia. The only way to know for sure is to see a doctor for diagnosis.
I once watched a martial arts program where the instructor claimed he could retract his testicles at will in order to protect them from attack. Afterwards he would of course be able to return them to the scrotum. If you can only detect one teste in your scrotum and think the other one is in your pelvic cavity then your conclusion may be correct. Try visiting a sauna or other very warm place to relax as I would expect that to have an opposite effect on the position of your testes to visiting a very cold place, and then see if you can ease it/gently chase it into the scrotum. If it doesn't then, yes.. see if your doctor can ultrasound/mri exam you to identify where the missing teste is and advise on risks of leaving it there (loss of sperm production from the retracted teste?) and risks of putting it right by surgery.
