LOL this reminded me of my camping days. As time went on we all got gf's, and then they'd bring girls along and every morning we'd find dozens of half full beer bottles.
US guys would look at each other and think "wtf dude" but as time went on, we found it wasn't us, it was the girls.
What we did one time is put an x on the label with a black marker of all our beers. Then the next day, count the halfers with x's on them. We'd find maybe 2, the rest were blank.
We accused the girls and they were all "it isn't US'....until we showed them the x's...lol (that didn't go over well, let me tell you, women do NOT like to be proven wrong lol needless to say there wasn't any tent sex that weekend hahaha).
From then on we said "you buy your own beer"......when it was theirs? The halfers dropped to almost NIL.....When it was ours? Ach, I forgot where I put my beer, no problem, grab another.......