Well, it depends. There are passive and active implants. The passive kind can only be scanned at a very short distance, say a few centimeters or even direct contact like an ATM card. Active devices could be made in lots of different distance categories, but probably with a corresponding tradeoff in size. If you want to find someone in the mountains, you are going to need something like a Lojack butt plug pushing out, I don't know, 20-30 watts? I suppose it could be lots smaller if it just sent out a coded ping every minute or so. If you just wanted a bluetooth king of thing, it might even be possible to have something that runs off the natural potential in the body.
And yeah, I guess the eyeball thing wouldn't work, but like you, I really don't want to know. Same with those fingerprint keyboards. It doesn't take any great imagination to realize that it probably wouldn't require the actual removal of the "part."
Anyway, there are, and always will be, really good reasons to like cash and barter. And really good reasons not to be "findable." (eg your .sig file!)