I'm curious as to why anyone would think or suspect that a missing testucle is in the sheath. Do you know it is there and not in the abdomen? Crypts may have the missing one just high inside the ring, just out of reach of the normal gelding procedure, or it may be somewhere in the abdomen, usually quite close at hand to a surgical incision made in the lower flank, or more like under the flank, in the abdomen next to/just ahead of the sheath. In rare cases the missing testicle can be much higher in the body--I heard of one horse that had it up near the spine. I have never heardvofbone being in the sheath.
Mind you--I have noticed that some minis seem to have a stage where the testicles seem rather oddly placed, almost against the back, upper portion of the sheath--I have noticed that in a couple of yearlings but by the time they were gelded at age 2 the testicles were in normal position.