Originally Posted by
WineCountryUA
Use of a non-compliant lav is allowed by crew in some situations, such as non-working smoke alarm or other conditions. That's what make this a difficult issue to have certainty.
This is also why the pilot/FAs have to be vague on why the lavatory is locked.
If the FA announced one lavatory is out of service due to a non-working smoke alarm, what's the over/under of someone rushing in there to light up/vape? I would put it at 5 minutes.