To me they seem to come in waves. I tried out Eco Lifestyle when it was available as a trial version a couple of weeks ago and during that my sim got abducted 3 times - and she wasn’t even in the scientist career, but it later turned out that at least three sims among her friend/acquaintances were aliens in disguise.
When I picked up my Cottage Living test save again afterwards, my sim in that save (who’s not in the scientist career either) got abducted 2 times pretty shortly after my return from the alien-filled Eco test. It almost felt as if the influx of aliens had spilled over to the Cottage save. So far my sim has identified three disguised aliens among her list of friends/acquaintances.