Comment 3 for bug 1248541

Found the problem - while AccountsService doesn't return system users when calling ListCachedUsers() it does generate the UserAdded signal when a system user is created (e.g. by calling FindUserByName()). The greeter then picks up this new user and doesn't check if it's a system user.

I think this is the actual cause of the LightDM user showing up - the daemon gets this user via FindUserByName and if there happens to be a greeter open at this time (probably racy) then the greeter will show the user.

lp:~robert-ancell/lightdm/system-user fixes this by ignoring system users when generating the user list for the greeter.