When I edit a mod in the CS then open Mash, a pop up tells me 'dates have been updated for xxx mod'. Usually this pop up happens once, but sometimes it keeps popping up whenever I open mash, why is this?

Well, with Mash's "Lock Times" on any changes to a ESP/ESM's Last Modified Time will be identified by Mash and reset to Mash's stored value.

Why you're seeing this I dunno without more info. Maybe an AV scan? Unlikely. Read-only attribute set? So Mash can't redate the file. It's a file you've modified elsewhere, somehow?