You might be right, maybe I changed the permissions, though I don't think this is too probable. We changed from a personal to group permissions, and all my users are in the same group having access to all projects.
I don't think it makes sense, though, to have someone subscribed to a project he does no longer belong to or have permissions to. Would expect all subscriptions to be removed when permissions are removed. Or at least he should no longer be displayed as "subscribed" in the overview.
I will keep an eye on this and get back here if I find something unusual! Thanks so far!