I had a similar situation play out in one of my households in my most GF save. After a sudden expansion of the family (semi-long story), I caved in and had one of the parents hire a nanny to look after the toddlers and babies for the day so the 1 parent that was home could get some work done. The nanny ended up staying over a day unprompted and then I finally got the "time is up" notice. Next day, the nanny showed up again but no one had called. I'm like, does the game think I need more help? lol Anyway, I had an emergency in another household so I had to flip over there for a moment. And, when I got back to his family again, the nannies had doubled--the original and a new one. I had my sim dismiss them both individually and they haven't returned unprompted again. IDK if this helps, but hope it was an entertaining story of what happened to me (regarding this issue). :D
