But I may never know exactly.

As my last resort I put a humane trap with a can of tuna. After a couple of nights, I got it.
I'm writing this from bed just after seeing that it went in last night.
Plan is to take it to animal services.

The incredible thing is on *several* of the days when the cat was in the garage at night I drove during the day. Sometimes at freeway speeds. One day I parked it all day in SF. So if it did indeed enter by being somewhere under my car, then it was being driven along everywhere!

I looked with a flashlight under my car. It's a Tesla Model Y so the underside is mostly flat. I believe it may have found a comfortable spot somewhere around the front right wheel well / suspension area. But I haven't been able to see it and confirm exactly.

This was me banging the underside with a broom hoping it would come out. Here you can see my best guess as to where the cat may have been living.