I didn't say he was 'in a cabin' he was fixing the public toilets in the hull deck Deck 0. Where the swimming pool and sauna were.
The lower spaces were watertight. However, to access the engine room, doors had to be opened. So a light would come on and the same signal would be seen on the bridge, that the door had been opened and then properly shut, afterwards.
Estonia had fifteen watertight bulkheads. This ensured its buoyancy.
Why would Sillaste make his way to the Engine Room if he was fixing the passengers toilets (at one in the morning?) unless he could see there was an emergency, such as water on the floor?