Grimrock

I've had a couple situations where a thrown weapon or an arrow/bolt has not been recovered where a monster is killed. However, a later monster of the same type, killed later nearby, will have an extra 'arrow' to make the count right.

I also note that in the snail room, killing a snail doesn't always result in snail 'meat' being left. However, later,another snail killed there will leave 2 or 3 portions of 'meat'.

Am I imagining things?

Snails typically drop 0-2 pieces of meat. As for the sharp projectile issue, I've never heard of that before.

I found only two situations where thrown weapons or arrows/bolts (-> ammo) are not recovered automatically:
1) The ammo sits on a pressure plate
2) You switched weapons because you ran out of one kind of ammo (i.e. fire enchanted arrows -> normal arrows)