i have had problems with cats here. There are some cats that just come hang around looking for mice, usually in the evening and early morning. That's fine, rock on. But i had a neighbor's cat (two actually, but one was worse) who insisted on being all around the pens, on top of the pens, in the duck pen when i had the door open for them to come up on the patio. i talked to the neighbors repeatedly, to no avail. i got tired of my birds being frightened all the time, and one even injured herself running in fear. So, i took the cat to Animal Control, after finding it yet again in our duck coop. i made sure AC had the owner's contact information, as i did not want harm to come to her. Since that time, the neighbors keep their cats inside.
i love cats and would never want to harm one. If you don't want to try to look for the owners, then a note on it's collar should convince the owners to keep her inside. Good luck!