It may well be outside, and perhaps when he went out alone.
Because you keep your eyes open for fossils and such, hopefully you will spy it on the ground, on a bush, etc. If it's anything like what happens to me, it will be when you aren't looking for it specifically.
That has happened to me with bird seed. Hopefully, it's not a new trick. If so, you will need fresh paper down all the time in order to put it back into his bowl. Sheesh. Kids!
Are you having to pay for the vaccine?
Hmmmm. That's a scary thought...that the squirrels know something, and are changing their behavior!
Hope you find the tags soon!
Love All Life, Thank You For Posting! :)