Saturday I thought it would be a good idea to take Rusty to the lake for dinner, so just before 7PM, I put him in a backpack carrier, and rode my bike 6 blocks down the hill to the lake, with half a can of cat food, and a spoon.
It was a little too noisy.
While he did well on the ride, and didn't hardly complain at all, before I even unzipped the carrier, I could see he was a little perturbed, but thought letting him out would help. We had set up on top of a small, fenced porch that was the entry to a rec center beside the lake, about 6 feet above the ground. Lots of birds come out at this hour. We had the local high position, and cover, and concealment, against the side of a closed building.
As soon as I let him out of the carrier, he flips into panic, evade, escape, and elude mode. I imagine losing an eye if trying to physically intervene at this point, so I calmly say "it's OK" repeatedly in my high pitched talking to cats voice.
Within 30 seconds of getting out of the carrier, he has upgraded his position to on top of a support joist underneath a two foot overhang of the first floor over the foundation.
I'm thinking, OK, this works, we can eat here.
And about five seconds after that he discovers a small hole, barrier bigger than a cat's head, that leads into whatever is behind the foundation wall.

I immediately know we are in for a longer night than I planned, and pin 1 hour after whenever the music stops as our best case scenario for recovery. The initial hope is maybe by 11pm, he crawls out the way he crawled in, and we go home.
Assuming he is not coming out for a while, I figure I can walk around a bit, and examine the perimeter of the building enough to be mostly confident that there's no other way out, and spot a clear human sized access panel that's secured with a small padlock, and four Philips head screws.
Before what looks like the last people for the night leave the park, I ask if perchance any of them have a Philips head screw driver I could borrow for two minutes. This person who I had never seen before says, "you know, actually I have an entire tool kit I don't need, and you can just have it." He gives it to me, and drives off.
It took nearly two hours to unscrew three, and a half of these four inch screws that were in two inches of the densest wood I have ever seen. The last half of the last screw stripped before I could finish, and I had to bend, and break with the crescent wrench, and the pliers that I insisted I didn't need.
After imagining all manner of worst case scenario for what might be inside this crawl space next to the lake that has a literal parade of rats, and raccoons come out at night, I decided to enter with my phone light after making enough noise, and repeatedly singing "Rusty.......Big strong Rusty......"
The space started with about four feet of headroom, which became three, and then two, and then less feet of headroom as you made your way towards the back of the building. There were multiple rooms, and a divider wall where he came in, but by dropping a string of lights from my bike down the entry hole from the outside, I could confirm he probably didn't get stuck between a wall, and probably came in on the larger main room I was in, and not the 18 inches of clearance section that was on the other side of the wall. All around the perimeter, and upper sections were tiny sections that I hoped he was hiding in, and just not responding to my calls. Crawling around looking for him would be insane. But before I left, I estimated that due to gravity, vertical distance, and tight spaces, getting out looked more difficult than getting in, so I built him stairs out of old paint cans, closed the access panel, and put the screws partially back in.
It was probably about 11:30 by now, and I was about 98% confident he was in there, and OK, and about 60% confident that if I went home right then, he'd find is own way before sunrise. But I also didn't want to leave him. I didn't want him to think I was abandoning him. I wanted him to understand we could go somewhere else, explore, and come back. The grand vision of an adventure cat.
My mind moves to what is a reasonable amount of time to wait? If I were telling this story to someone, and I said I went home at 8, what would they think about how much I cared about my cat? I figure midnight is a time I can live with, and anything beyond that, this cat has seriously decided to be on his own, and does not trust me one bit. Midnight comes, and goes without notice. One of the many inner voices says 1AM, stay till 1, then you can go home. 1AM arrives, and I stick my face in the hole one last time to call "Ruuuustyyyyyyyyyy....", and I see a single whisker extending an inch beyond that support joist. Now I know I can't leave. About five minutes of coaxing later, and I see the tip of a nose, and maybe 15 minutes after that, I see his face for a second. But he is still hiding in this thing so much that I'm not even 100% certain he isn't stuck on something. By about 1:30AM, he has his entire head, and one paw out of the hole, and I'm thinking he has to be physically able to get out, he's now just observing. I considered trying a snatch, and grab, but thought that would be the quickest way to get him to flip out, and lose trust, so I placed a scoop of cat food on the joist in front of him, and watched him eat it slowly. By about 1:45 he climbed out, hopped down, came towards me, and I was easily able to pick him up, and put him in the carrier.
We finished the can of cat food outside in more familiar territory 6 blocks away.
As soon as the smell of cat pee is gone from the backpack carrier, we will try exploring again when it is much quieter.