I would not feed a strange cat, of course she'll turn up again for a another free meal. I would monitor any cat hanging around but unless they were hanging around 24/7 or looking sick/ill I would be carefull about accidently catnapping a strangers cat.

But while you are concerned about her, I would suggest a paper collar, it will fall off if it is not removed and taking her to the vets to get scanned for a microchip.
http://www.lostmycat.com/foundcat.html

If she returns WITH the ribbon & note - chances are she is homeless.
I disagree, if she returns with the collar she could be lost or her owners could be on holiday and the pet sitters didn't notice the new collar. The collar test is only any good IF someone rings you.