I can only hope that Binance has a clear-cut policy on this. But I suspect there is none. The platform can freeze your fund or lock your account all of a sudden and without reason. When asked for an explanation, they can give as generic and shallow an answer as "we detect a suspicious activity in your account". Indeed, there are triggers. But even these aren't clearly pointed out one by one. It's not once, not twice, not thrice, but many times that accounts were frozen pending proofs of source of fund with the owner not knowing why.

Also remember that proofs can sometimes be very hard to provide. Binance could even go as far as ask you to prove that you indeed own the wallets from which all your deposits were made. Oftentimes, Binance and all these centralized exchanges just want to hostage your money.