I think it is somebody who works on the planes, an insider. It might even be a mechanic.
I´ve heard from an American colleague, that, when Lufthansa laid off their own mechanics in the US to let UA
do the maintenance work on LH
planes, some LH
mechanics commited acts of sabotage and the whole lot got blacklisted due to this, I don´t know if this is true.
Does MAS have a labour dispute?
On the other hand, about 15 years ago, there was a series of sabotages at THF
in Berlin, somebody who knew what he was doing e.g. cut deicing boots on planes or stuffed cleaning rags into the oil cooler air intakes of helicopters, so that the pilot wouldn´t be able to see it during his walkaround. AFAIK, they never caught the person. The sabotage stopped as suddenly as it started. Airport authorities assumed that it might either be a still working airport employee or somebody who was working for the USAF
, when they still used this airport, but got laid off a few months before.
Any airport has security holes and if somebody knows how to move on an airport without attracting attention it is very hard to catch him.