ADVENT Container - Loot recovery?

I have seen these before and opened them a number of times.

On each occasion the box is visually empty - as in your screenshot. But the audio of the solider says "there is some loot here" and there is a sound of items been scooped up.

I just did a VIP extraction mission on L/I - and I noticed I had an "extra" item at the end. I had 2x repeaters, an elerium core, and a scope. That is 4 items, but I only ever get a max of 3 from an alien drop.

So I can assume at this point the 4th item came from the chest. This would imply you get a "random" item from the chest at this stage...

Edit: I can now confirm - you do receive "random" items from the container. I just completed a mission, and received an extra "Laser Sight" and "Hair Trigger" mods that were extra to what I had picked up in the mission.