When Rosso is sound asleep we can pick him up completely outstretched in what we called the "dead rabbit" position and he doesn't stir.

My cat is very lazy and can sleep for hours! Sleeps like a log and when dreaming she pretends she's running
Probaly running after mice... If she had the energy

Well they know you aint gonna hurt them, there's not other (potentially dangerous) humans nearby, and no dogs/cats that are likely to attack them. The loud noise may have made them think an intruder had entered the house. Mine legs it behind the couch when I let people in. Had a hard time finding him the other day, though he'd somehow ran outside.

Mine does this cute little thing where he drags himself along on his front paws (whilst awake) towards me when on my bed. His back legs seem to work perfectly fine - he just seems to like doing that sometimes. I think cats do dream and have nightmares, as mine seems to have had nightmares and I've carefully woken him up.