It's unlikely to be a glitch in Norton, since it's happening on your two systems but not to Norton's millions of other users (note the absence of "me too's" on this thread). Usually, these things--when not malware-related--end up being something in the user's local hardware/software environment, Internet provider, etc. I'd still suggest--at a minimum--running LiveUpdate on each machine until it tells you you have all the latest updates (if you haven't done this already).
But after that, if Norton and malwarebytes aree both giving you a clean bill of health, and you don't have any other indications of malware, and you don't feel like going to a removalist forum "just in case" -- and given what you say about it not happening all the time -- there isn't likely to be any harm in "wait and see," if that's what you prefer.
We'll be here, if you change your mind.
V/R,
--DistEd2