When he hit the slide release I heard the round slide into the chamber. I am a glock nut. I own quite a few of them. Glocks have very distinct sounds, when they fire, when you hit the slide release....ect. You can tell by the sound almost if you have a round going into the chamber. I didnt hear the "clank" you normaly hear when you hit the slide relase on an empty glock.
As for the Glocks going on when holstering it is the fault of the student, not the gun. When I went to SRT school in the Army we used Sig 228s and 226s, in my class alone we had three guys shoot themselves holstering there weapon. Lession? Keep your fingers clear of the trigger area.