What might have happened is that TCX had gone to Boeing with a problem which they might have found and Boeing suggested a fleet check on the rest of the TCX fleet.
This is only a guess, as l am unaware of Boeing issuing a Tech Bulletin on the world 757 fleets, but l might be wrong and have missed it.
I know a problem was found on one aircraft in TFS on Friday afternoon, but l do not know if this is linked to the Mahon inspection.