I do this a ton, and with a couple of different variations. I'm not sure if it's an autism thing (I'm still trying to figure out if I am) or something else. Staring off into space or at interesting patterns. Definitely getting lost and trancing out at complex patterns in fire or water or leaves, or lately textures on things. Often those ones aren't stressful at all really, they're just getting distracted and caught up in interesting sensations. I do that a ton with music, too. Of course there's also definitely shutting down from stress or fatigue, which I suspect may look similar from the outside but the focus feels very different instead my head. And when I'm doing complex computer things (I'm a software geek professionally) I'll often sort of trance out for extended periods thinking about the problem I'm working on, or even not really thinking analytically about it (which sometimes gets overwhelming) but just getting lost in my internal understanding of it, sort of the same way I do with water or fire patterns but applied to that abstract system.
Something like that, anyway.