Checking!

After the starboard engine got a bit hot on Sunday night, I gave it a good check over.  Oil and coolant in the engine loop are fine.  The seawater strainer is fine too.  There is a very slight leak on the water pump, but that's just a few drips (I'm already waiting on the engineer coming to fix that).  Belts seem to have retained proper tension.  Seawater strainer (open in this picture) is completely clear.  Nothing looking worrying.

I'll stick with my original hypothesis that something temporarily blocked the intake, and slowing down reduced the intake suction enough to for it to come off.  I'll keep an eye on it though.