HELP can't take the cat to the vet unless I know for sure he needs to go.?

He is a neutered male about 8 or 9 years old in perfect health, had a physical about two months ago and is fine, no health conditions, no medications. Just now I discovered a patch on the inside of his foreleg that is completely hairless, about an inch and a half long and about three quarters of an inch wide. The skin looks normal, pink and healthy, just no hair at all. I touched and pressed it and it doesn't give him any pain, he doesn't react at all and he's acting normally. No zits on the patch of skin, no scars, no anything. What could it be? He doesn't scratch, has no fleas, is an indoor cat only, doesn't wear a flea collar, anything. Does anybody know what this could be?