The muon’s charge excites electrons as it passes through an atom’s electromagnetic field. The camera is detecting this trail of ionisation as the muon passes between the atoms of the sensor (and these sensors are very good at detecting excited electrons). The muon does NOT need to decay or to strike the atoms of the sensor directly in order to be detected.
In open air at sea level, you would expect 1 muon to pass through any square cm of ground every minute, on average. With a sensor measuring 2-3 sq cm, oriented correctly, and exposing for a long enough time you would certainly expect to catch a few.
Unlike x-rays or gamma radiation, muons can pass through several km of dense matter and penetrate deep inside the earth before they decay. They can pass through solid lead. Direct particle collisions are rare but more likely when passing through large amounts of dense matter. The ionisation process can also reduce the speed and trajectory. Muon tomography works by comparing how much the muon count has been reduced compared to an expected background level.
The practice of capturing muons on camera is quite well established, see for example https://pmc.ncbi.nlm.nih.gov/articles/PMC10220736/
