This is of course why you need a digital camera - take a picture first, then approach, then you have something to post and/or ask around about....

Older tracker-chips needed a close-up reader, I wonder if RFID has caught on enough in that space - if they have, small and cheap readers are starting to appear [nokia has an add-on shell for one of their phones, for example, that includes a short-range reader], it might be interesting to have a "nearby cat tracking" system for your yard :-)