Marco, I can reproduce that with 100% reliability on my 16.04 beta (with all the updates installed). When I see the auth dialog (i.e. after locking the screen or returning from sleep), I hit ctrl+alt+F6 ctrl+alt+F7 and gain access to my system. During the process, nothing at all gets added to /var/log/auth.log

user@host:~$ gdbus monitor --system --dest org.freedesktop.login1 & gdbus monitor --session --dest com.canonical.Unity
[1] 26646
Monitoring signals from all objects owned by org.freedesktop.login1
The name org.freedesktop.login1 is owned by :1.4
Monitoring signals from all objects owned by com.canonical.Unity
The name com.canonical.Unity is owned by :1.14
/org/freedesktop/login1/session/c2: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Session', {'IdleHint': <true>, 'IdleSinceHint': <uint64 1459227675096697>, 'IdleSinceHintMonotonic': <uint64 11646057942>}, @as [])
/org/freedesktop/login1/seat/seat0: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Seat', {'IdleHint': <true>, 'IdleSinceHint': <uint64 1459227675096697>, 'IdleSinceHintMonotonic': <uint64 11646057942>}, @as [])
/org/freedesktop/login1/user/_1000: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.User', {'IdleHint': <true>, 'IdleSinceHint': <uint64 1459227675096697>, 'IdleSinceHintMonotonic': <uint64 11646057942>}, @as [])
/org/freedesktop/login1: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Manager', {'IdleHint': <true>, 'IdleSinceHint': <uint64 1459227675096697>, 'IdleSinceHintMonotonic': <uint64 11646057942>}, @as [])
/org/freedesktop/login1/session/c2: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Session', {'Active': <false>}, @as [])
/org/freedesktop/login1/seat/seat0: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Seat', {'ActiveSession': <('', objectpath '/')>}, @as [])
/org/freedesktop/login1/session/c2: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Session', {'Active': <true>}, @as [])
/org/freedesktop/login1/seat/seat0: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Seat', {'ActiveSession': <('c2', objectpath '/org/freedesktop/login1/session/c2')>}, @as [])
/org/freedesktop/login1/session/c2: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Session', {'IdleHint': <false>, 'IdleSinceHint': <uint64 1459227691145076>, 'IdleSinceHintMonotonic': <uint64 11662106321>}, @as [])
/org/freedesktop/login1/seat/seat0: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Seat', {'IdleHint': <false>, 'IdleSinceHint': <uint64 1459227691145076>, 'IdleSinceHintMonotonic': <uint64 11662106321>}, @as [])
/org/freedesktop/login1/user/_1000: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.User', {'IdleHint': <false>, 'IdleSinceHint': <uint64 1459227691145076>, 'IdleSinceHintMonotonic': <uint64 11662106321>}, @as [])
/org/freedesktop/login1: org.freedesktop.DBus.Properties.PropertiesChanged ('org.freedesktop.login1.Manager', {'IdleHint': <false>, 'IdleSinceHint': <uint64 1459227691145076>, 'IdleSinceHintMonotonic': <uint64 11662106321>}, @as [])
^C
user@host:~$