Where does the hotel keep the deposit room card after confiscating the deposit?

Front desk.

According to personal experience, when you stay in a hotel, the room card is usually given by the front desk. After staying at the hotel, you need to return the room card to the hotel front desk in full and complete the check-out procedure. It can be seen from this that the hotel's room card that confiscated the deposit was also managed by the front desk, so it is at the front desk.