What the AMM says:
D. Operation of the Lower-Deck Mobile Crew-Rest (LDMCR)
Compartment-Ventilation
The ventilation and individual air supply comes from the cabin air-supply
ducts for areas 5 and 6. It flows through the LDMCR isolation-valve 216HG.
From the isolation valve, the air flows through the air heater (Ref.
21-63-00) into the LDMCR.
The flow sensor measures the airflow through the air heater in the
ventilation duct of the LDMCR.
If the airflow into the LDMCR is not correct the flow sensor gives a signal
to the buzzer and to the LEAVE MCR signs.
..I don't suppose it's necessary to rush out screaming, but if you were sound asleep, the buzzer will wake you up. Without fresh air supply into the MCR, and with 8 bodies respirating, the CO2 levels would definately be increasing!