Here's the thing. This is a busy passenger ferry. Usual crew and staff. The EPIRBS were affixed one each side of the bridge in a float-free bracket with a Hydrostatic Release Unit. As one might expect with a big ship carrying up to 1,500 people, regular standard inspections are carried out. MV Estonia had her buoys inspected as being in full working order by the ships electricians and there is a log of it. The HRU unit is triggered when immersed in up to 4m of water, hence one on either side of the ship depending on which side it capsizes. To avoid the possibility of a false alarm being sent (for example hooligans or for battery saving) they are switched off if in port for a longer period than a few days, for maintenance or whatever. The fact the free float automatic buoys did not emit a signal, despite being apparent immerse, and the Rockwater divers retrieved one of the HRU's from the empty bracket cage, suggesting that either someone removed them manually and switched them off or the electrician/radiographer who inspected them didn't do his job, to check they were active and ready to activate in an emergency, which seems unlikely.