I've heard the military's solution to losing keys in their pool vehicles is to stash a functional key with a transponder under the dash and then just issue cheap replacement keys. It doesn't help with someone hotwiring the truck but I doubt anyone would find it.
I've heard the transponder itself does not need to have a battery. Is that true? I'm not talking about the remote locks on the key fob. The Ebay keys don't look like you can replace the battery on them.