"Stomping ants" appeared quite early on in my games, too, but has reappeared only rarely. It's the same with when they decide to water the plants on their own. Obviously, you've got to solve the relevant puzzles before they can do that, though. I think some actions are just chosen infrequently so you need to be lucky to spot them taking place. The most recent "new" action I've seen is "Sorting mail", which I first noticed a couple of days ago. At the end of that action the peep puts a letter in the shredder. Since the shredder only appears in a L3 office, I assume the action only appears once one in bought.