In case this happens again, the usual cause is a game bug through which a car that your sim or household never owned themselves gets sold, but your household gets credit for the sale. If using NRaas SP, then a Household Options > Accounting Report can help reveal the sources of mysterious credits and debits. if the report gets too full of stale information to be useful after a while, it can be cleared out by way of a command on the same menu, a bit further down.

I don't see it happen very often and usually just shrug and take the unearned money away. I've never seen the funds correct themselves after the next loadup, but maybe I've never been patient enough to wait for that.

Just for inactives, I think. That sounds like it would have been quite a gift though. Do they really transfer high amounts around like that?