Basically Infantry sections drop brens, even when they are only carrying 1 weapon and there are still 2 squad members left. I am not aware of any other squads dropping weapons when they still have enough squad members alive to carry them.
I am not sure if this also applies to Infantry sections dropping other weapons too, I never get piats and I haven't used the tank hunter Infantry sections.
I have included a replay and the times that it happens so you can see.
19:45 an infantry section going from 3 to 2 men with only 1 weapon drops a Bren
30:45 an infantry section going from 4 to 2 men with only 1 weapon drops a Bren. The 2 man squad picks up the dropped bren and it disappears. Picked up message shows but the weapon is not on the squad.
replay:
http://www.coh2.org/replay/46258/bren-drop-bug
