I've checked my account registers today vs my registers at my bank. I use file-import for that. And an inflow of $645.20 appeared with the very unpoetic payee name "Rus Ebt/Rbt". Neither my husband nor I have any idea where it's coming from.
That's the beauty of YNAB: you worry so little about your finances, you don't even realise when you are due money.
It could also be someone who made an error in the details of a transfer I guess. Who knows!