I know trenches were not used as much in WW2 as they were in WW1, but after the trenches were "done being used" in either fights were they immediately filled in or were they just left as they were to slowly be filled in over time?

Secondly did people go in and steal all of the various metals from the barbed wire I between the trenches or even anti-tank obstacles, or did the different military's go back and collect their items?