They sadly weren't in a board game (that would have been a good photo-op). We looked behind the printer too - and to no avail. They weren't in the boxes either. Or behind books in the bookshelf. The drawer was a suspicion, but I realized they all were closed, and unlike some drawers, there isn't actually open space in the back where they could crawl in.
What they did is much weirder.
As anyone who has visited our house knows, we keep a double mattress crammed under our queen-size, to comfortably accommodate visitors:

That is jammed extremely tight; it takes about a minute of see-sawing it back and forth to get it to leave. Nevertheless, that is no obstacle to COMPRESSIBLE NINJA KITTENS:

Believe us, it took us a while to figure out that there might be about a thumb-height of space between the top of the jammed-in-mattress, the slats, and the actual mattress.


Anyway, they both clearly like it there! Some other cute shots:
