In my flat I had a normal hook for towels in the bathroom. And this one. It's obviously just a part of whole hook, but other hooks are solid (can't be disassembled). I found that it's probably Gio Bella Riete collection, but didn't found any possible match. Examples of other accessories from this collection:
Thank you! I agree it's the best guess. It was on a face level on the edge of the tub. So it could be used both inside and outside of the tub. But what makes me doubt: most pivoted mirrors have the moving part between two fixed brackets. And of course no similar images on the internet.