I just noticed this today, haven't seen it before. It's not the edge tab cause I turned it off for the moment.
It just vibrates when I touch it. Doesn't do anything when I hold down on it either.
One thing I really miss with an unrooted phone is Titanium Backup!
One of its cool features is sorting apps by date of download or update. This mashes it easy to determine which app may be presenting this or causing problems.