I know you've surely exhausted yourself with searching -- but let me mention a few places you might not have thought of:
- When my Clyde was tiny, he found a two-inch gap in the corner of my kitchen cabinets where the toekicks didn't quite meet. I tacked in a shim to close it, but then saw him STILL reaching in somehow! I got a mirror and held it under the edge of the cabinet, and amazingly, there was another small gap that led UPWARD into the empty, counter-topped corner. Similar gaps can occur in soffits and indirect-lighting enclosures, too.
- Some undercounter cabinets have holes that are too large for the pipes, and this creates openings to the space between the cabinet floor and the subfloor. I would doublecheck every spot in the apartment where a pipe or duct of any kind enters. Make sure the ducts are all secure, too -- if there's a loose one, it's possible Teufel managed to get in behind it and can't get back out.
- Is there a trash chute in your apartment? I'm sure you thought of that if there is...
- Do you have a piano? I had a kitten slip inside an old upright once.
- A couple of years ago, I set up a bedroom in my parents' house for four little foster kittens. I thought I had kitty-proofed the room, but within moments after I brought the kittens in, all four disappeared! In twenty minutes of frantic searching, I eliminated every possibility... so I tried the impossible. In setting up, I had shoved a huge, heavy dresser flat against the wall, leaving a gap so narrow that I couldn't even slide my hand in between. There was no way in the world they could have gotten back there! Nonetheless, I shone a flashlight into the gap... and sure enough, no kittens.
But there was nowhere else they could be! I carefully edged the dresser away from the wall, and only then could I see that there was a two-inch gap between the back bottom edge of the dresser and the floor. I had to pull the dresser ever so gently out farther and farther until I could finally lie flat behind it and shine the flashlight in -- and there they were, four little fluffballs huddled together in a space not more than two inches high. (They were all precious, and they all found good homes within weeks.)
Finally: if Teufel really did get outside, please bear in mind how incredibly cute he is! I can't imagine he'd remain a stray for long. Most likely, some other family has taken him in, and that's why he hasn't been able to come home. So although I know you miss him desperately, please don't imagine the worst... he's probably safe and warm and being cared for.
Sending heartfelt hopes for your baby to come home soon!