The Room Becomes Occupied When No One is in the Room

Does this happen ONLY when someone walks past the room in an adjacent room?

Yes, it only happens when someone walks past the room.

Then check for a Bleeding Sensor. If it is a Bleeding Sensor then, preferably, reposition the sensor (or tape the inside of the lens) so it doesn't bleed into the adjacent room. If that is not possible, then try enabling the "Bleeds into" checkbox in the Beam Ins Menu and click the room it bleeds into.

No, it happens when no one is around.

Then, the sensitivity of one of the sensors in that room is too great. Find which sensor turned on the room.

Is the sensor an Area Sensor?

Yes, it was an Area Sensor.

Then try decreasing the sensitivity of that Area Sensor in the Sensor Sensitivity Menu. If that does not work, you will have to decrease the Analog Sensitivity of the Area Sensor.

No, it was an Entry/Exit Sensor or a Getting Out of Bed Sensor

Then, A sensor is not working properly.