Since I know everyone is biting their nails to hear how this cliffhanger ends...

I finally got a chance to talk to the neighbor today. Thanks to the advice I got here, I went up and started from the start rather than jumping to conclusions. I told him that while mowing the backyard, I found a cat collar for "Ashley", with his address, and does he have a cat by that name? He said "Yes, that's our cat - the black one."

So it turns out the collar showing up in the backyard was just a coincidence, and that Ashley is just fine. She slipped out of her collar somehow, and I returned it to them today.

Thanks to all who replied,
GM