When was the last time that you looked under the TR driveshaft cover on a Check A???
I do, quite regularly, on that type, because it says in the RFM pre-flight "full" checklist to check for grease leaks from the bearings.
I also check the "jiggle pins" are loose, as intended.
And as soon as I saw those photos (in fact one in particular), before I heard any rumours on the cause ........ I went in to work specially and had an extra special good look on the aircraft I fly most, although I wasn't sure what I was looking for.
Having said that, there is no reason for a pilot to have a rag up there while he's looking - there should be nothing to wipe up. If there was, I'd be calling for engineering advice.