Just put your hand or a couple of fingers over the area where the magazine goes, over the bolt probe, ideally without the mag in place. Your hand /couple of fingers will 100% detect a leak and its severity better than Kleenex in this region. Many instances with airguns where you don't want to use your hand, but in this area its safe. I used to do Kleenex / TP, but then last few times I was like, why am I using my visual sense opposed to my sense of touch? Lol
A little wisp of air shouldn't contribute much to the guns report, but a violent one certainly may. If you could isolate a muzzle report from your leaks report, if it were a wisp it would sound quite literally like a mouse fart. If it were a larger violent leak, it would be violent and air would escape fast enough to vibrate / shock enough atmospheric air to become loud, akin to having to muzzle blasts.
-Matt