I've heard that looting is instanced - so friends or other players whom I can see find stuff in containers that were either empty from my POV, or else completely different items = which I already knew about.

But, I've noticed something weird. I usually only loot green or blue items, and even though I rarely see another player in this one location I often visit, if I go back to a container that I left full, l most often find it empty. So its contents have disappeared, after I've taken my character's eyes off it (never while looking at it). This has happened more than once, and I proved it to myself last night in the Greedy Gut when I deliberately left a container full, walked away and returned about 30 seconds later to find it empty. While it's possible there was another player whom I didn't see, I doubt they would empty that one container and none of the others - I'd have seen them at some point.
An example is the inn in Kvatch. I checked all the containers upstairs, found nothing worth taking, and then noticed all of them were empty as I went back to the stairs - but again saw no one else. There have also been containers that had "steal" on them but when I've gone to look inside they've suddenly gone empty...

So if it's because the players are also instanced, why does that affect the containers in MY instance?

TL;DR How is the loot disappearing if no one else is there??