My dogs paw/knee wart?

You really don't give a very good description, but they sound like either elbow callouses or, if they are lower down by his foot, his carpal pads. The general rule of thumb is that if something is symmetrical (the same on both sides of the body) then it is normal. Warty growths, tumors, wounds, etc are almost never ever the same on both sides (and usually don't come in pairs anyway). This is probably why the groomer and the vet have never mentioned them. I wouldn't worry about rushing him to the vet, but if you're curious then ask the vet next time you're in and I'm sure they'd be happy to tell you exactly what they are.