Smoke grenade disappears after smoke has end.
Weapons, remaining on battlefields after battle, give us effect of the presence. This is one of the important things. We start to "believe" in picture on the monitor.

In long DM etc., this unneсessary parts must be removed for take good game perfomance. But after some time, 15-20 min, in example.
Or, better way - give mission-makers some sort of functions like "automatic battle junks collector" (in a style of "Garbage collector" module). In options for this module give setup garbage classes to save/remove from the field.