EFI automounts on startup
Since El Capitan this is the behavior of Workgroupmanager for local accounts if you enable to manage internal harddrives to be only readable for some users. It doesn't matter which user logs in. If there is only one user managed, the EFI partition will show up on start for all useres. Disabling this in Workgroupmanager again hides the EFI partition, too. 
Since Workgroupmanager was lastly updated on Mavericks it is still possible to manage e.g. USB shares to be only readable without having to install a OSX Server, as there is no on board solution for managing user accounts more differentiated than with System preferences.