Unidentified object photographed over Barcelona — strange irregular movement

I photographed this in Barcelona in broad daylight. There were several of us watching it. We initially thought it was a balloon or a black plastic bag. Its overall trajectory was fairly straight, but its movement was irregular: sometimes faster, sometimes slower, occasionally rising and dropping, almost as if it were floating. We watched it continue toward the mountains until we eventually lost sight of it. I’m not claiming it was a humanoid or anything extraterrestrial — I genuinely don’t kn...