I think they usually ask for another bank account/IBAN to send you your balance via SEPA, before they close your account.
Unless they deem your money is a product of "illegal" activities (whatever that means)... in that case, they will seize it.
A bank will usually not straight up seize your money without a court order, that'd be too much off a legal headache. Simply closing the account is less of an effort and thus cheaper.
If you fail to provide another bank account they will send your money to an escrow account though, in which case the legal headaches to get your money becomes all yours.