Ei131 Smoke Alarm Battery - where is the thing

After poking at it for a while, trying to change the battery on this alarm, and search for online manuals - I was frustrated that it didn’t work either as other alarms on the net did, or as the text on the casing suggested.

However the trick is to pull on the tab on the side - which reveals a battery compartment in a little drawer. Not obvious when you are looking for a catch to pull the top off or hinge it out or similar.

This is for the ei131 type smoke alarm. The tray is a nice little idea, probably safer than alternatives, if different enough from common usage to confuse people. Perhaps some more indication on how to pull it - or in the manuals would help?