Yipes! I meant to address #74 to you and not your non-hot sister/brother.
Why not let the cat out to chase it off?
We had a similar problem a few years ago. A cat was showing up at night, and our male indoor cat would go crazy. The straw for me was our cat’s marking the basement window where he was seeing the other cat.
I thought I had cleaned all of the “marker” urine and treated it with enzyme formula, but apparently I did not get all of it. Next thing was that our kitty, who apparently could smell that urine in the basement, elected to urinate into the heating duct in the living room that was above the previously violated window.
I kept smelling it and then going back to the basement to clean some more. It took about 2 weeks before I realized what had happened. The smell was sometimes stronger and sometimes weaker, so he must have been using the heating vent multiple times.
So it took me a long time, but I cleaned and treated the dang vent. Well, the next time that cat showed up, we let our cat out to chase him off. Worked like a charm.
Sorry, but allowing your pet to go chase a strange cat off is foolish. I would never ever allow my indoor cat or dog to have contact with the feral cat I feed. I have a German Shepherd and I would be distraught if the feral bit or scratched her. We have a high incidence of rabies in our area in the raccoon and fox populations. Also the local fox have an outbreak of scabies going on. My feral catches, beheads and eats squirrels. I would hate to see what he would do to one of my pampered, trusting pets.