Once the time has come for the bookmark notification to appear, the notification icon and its colour are similar to the one used for message notification. It would be much more clear that “that’s a bookmark notification” if it was placed somewhere else near the profile picture. (new position: bottom left, for instance). It would also help if the colour was different from the existing one for PMs.
Here you see 2 green notifications, but you cannot tell if it is a bookmark or message one.