I went back and took some daytime photos of where I found Tucker. I first saw him come out from under this car, near the garbage cans:
When he got spooked, he ran in the side yard of this house. We had checked this yard many times because of all the junk around it.
We think he'd been living in this garage (with broken door) for at least a week. Though we had shined flashlights inside and called his name, he did not respond then.
Tucker has a vet appointment today to get checked over. He is eating pretty well and is in a good mood. The cats are still separated and we'll take it slow in re-introducing. The hard part is over--we can deal with this.