I've always been given to understand that if a scan is run in the Administrator User Profile (it can't be run "as Administrator" can it?), it scans the entire PC and I've seen evidence of that in the past.
Since the latest Win10 upgrade, it appears that the scan is only scanning the User Account from where it was started. That's definitely the case with SUPERAntiSpyware - it's scanning at the moment so I can't check NIS.
I'll run it across a few users as soon as I can and will post back but if someone could comment in the meantime that would be very helpful.
Thanks
PS I remember when you could install a new program - I hare the word App - and you could choose whether to install it for all Users or select a User. I think that's gone now - it's along time since I installed anything that I didn't want for all users.