I just wonder: How is it possible that a user who is not member of any other site got an association bonus on this site?

@MartinR It is possible to hide the sites that one is active on from one's profile. Just looking a bit on other sites of the network, I have found what looks like profile for the same user on Physics and Chemistry. Note that these profiles have no activity, so the bonus must have come from activity on another site.

By the way, I noticed that the only activity of that profile was to upvote something and award a bounty on the question of another user. This looks a bit like some of the recent issues on MO, so it might be worth checking it out, and maybe consult the moderators from MO.

@MartinR I hadn't noticed that, good catch !

This keeps getting weirder : on this user's Biology profile (which I incorrectly labeled as Chemistry above, sorry for the confusion), there is a link to some Vietnamese (?) website, where I found a link to this blog, which seems to be the blog of a user currently suspended...