I’d forgotten I started this thread (not a regular visitor here) until I saw the signs again at Edinburgh yesterday - which I think solved the mystery.

They seem to be aimed at Dublin-bound passengers arriving from Beijing. There was a poster at the top of the stairs down to passport control (coming from gate 5) that was headlined as important information for such passengers; unfortunately I was in a a rush to try and catch a tram then train so wasn’t able to read it, but putting 2+2 together I assume that everyone is deplaned on arrival from Beijing, which means those going on to Dublin need to clear immigration and pass back through security to continue.