I remember talking to another Glock armorer who supported something like just under 100 G21's for his agency. His agency is located in the US where wind & blown dust, dirt & sand are not unknown.
Ever look down the open channel in the grip of a Glock?
That's how he presumed that a small chunk of grit/sand entered inside his trigger housing mechanism and lodged itself under the connector, behind the end of the trigger bar. It could have entered via the gap in the rear if the slide, I suppose, depending on how his holster was built or whether his G21 was exposed to wind gusts when it was in his hand. Dunno.
The thing is that he discovered the problem during a range session when he found that he couldn't pull the trigger far enough to the rear to release the striker.
He said he tried pulling the trigger hard enough, enough times, that it seemed to have resulted in a burr in his connector.
Periodic cleaning & inspection for normal function is a good idea under normal conditions. It's an even better idea if a gun is carried under harsh, adverse or extreme conditions.
Then there was the Glock armorer who discovered rather fine grit and sand inside his spare magazines (which, if I recall correctly, came to his attention as the result of some feeding problems when using those spare mags that were carried on his belt in his everyday activities ... where dust, grit and whatnot could be carried upon the wind). His suspicion was that the debris had gained entry to his mag bodies via the numbered windows in the back of the mag bodies. Some mag carriers expose a number of these windows, don't they?
In the Sig armorer class we were told that a pistol is NOT clean unless the magazines are also clean. I tend to agree.
But that's just me ...
