how did they get there ?

@ieatgreenblood  No. you are not alone.  That's just the way the game works.  It will use some of your playable sims (when you aren't playing them) to fill out the crowd at various places like the gyms and bars and such.  It will use them as buskers, and people on the street.  It will use them as Day of the Dead Celibrants and venders at the Flea Market.  It will use them as Caterers and Bartenders at catered events.   Lots of other ways too.  But when it uses them in some of the those roles, a lot of the time it doesn't send them home so the next time you load their lot, they may be somewhere else.  Usually, they will return to the home lot almost immediately but sometimes not and you have bring them home by clicking on their greyed out portraits.