This one appears during the chapter Route Kanal in Half-Life 2:
It's active, like all the backgrounds in the game. Essentially the chopper flies by and all the bottles and boxes in the water below it are blown away by the wind from the rotors, all simulated by the physics engine. I restarted the game like three times the first time I got this one just to see it again.
Funnily, I later checked the source file out in the map editor, and it's all faked. The props are moving because of scripted physics explosions and are just timed to go along with the helicopter. Not that it matters, all choppers in the game move along a scripted set path to some degree, but it wasn't
quite as cool as I thought that first time.