I forced my X to get his own bank account this past February. Within the last year he would often ask me for a blank check (which is needed for cash advance). I would give him one, never thought much of it. *We don't share an account anymore*
You know those mobile banking apps that deposit checks if you just take a photo of them? He could do the same thing with your blank checks. Even if you wrote VOID on them, if he was handy with a computer he could create a mockup pretty easily.
Friends of mine had thousands of dollars stolen from their account when some guy photoshopped their account and routing numbers onto a counterfeit check from a multinational company.
I know this is a PITA, but I would change account numbers if you haven't done so already.