It's part of the new attraction system.
Do you have a maid by any chance? My sims all get that popup when the maid comes, regardless of if they are on the home lot or not.
No, it's not with staff or anything. It actually happend on vacation. I have build a 'hotel' for my sim to stay and he got that message when he was writing one of his masterpieces. It actually happend all the time, around the clock, whenever someone noticed him writing. Say it papparazzi or ordinary people from France, they almost all popped up with that message.
saltpastillen is right. It's the attraction system. You won't get a boost to the relationship but you'll get a moodlet while talking to them about being around "an attractive Sim". The person who said it may try to call you up for a date or send you gifts in the mail like the omniplants you've been getting or something else from decor.
So that is that omni-plant weird bug? Things seem to fall in place finally. Never thought those two things that are new to me are connected this way. But now you've said it: my sim came home with even more packages in his inventory which I haven't asked for.
I assume 'return to sender' is not an option?
And the moodlet I've noticed. Kind of being around attractive sims. I just assumed NPC's would have the 'attractive' reward unlocked somehow.