I had aliens seemingly spontaneously reveal themselves throughout the worlds once. They remained un-disguised in HH-management, too, so I just deleted 'em all. I assume the new NPCs the game generated included more aliens, but they kept their human forms on, & haven't revealed themselves yet.
As Cupid suggested, I suspect I must've taken my sims to a bar during Alien night, though I don't remember doing so. I wonder if visiting a CL festival via one of the Karaoke bars is what did it? Seems possible.
Hope you too are able to avoid an unwanted re-invasion!