inside beep when car parked and turned off

today somethin new happend to me.

i googled about it but can not really find anything beside smart key related stuff buth mine is turned of and still you also get a beep outside that i do not.

anyway what happens is the following.

the car was locked and parked.
but the beep you also have when the seatbelt reminder is on when driving was beeping ( at that same speed really fast beeping when you do not put your seatbelt on after a while it starts to beep really fast )
i unlocked the car and opent the door and it kept beeping... now ofcourse i can hear it even better.. it was indeed the inside beep....

i closed the door and wanted to lock the door again but before i could press the button it locked itself....

i opent and locked the car again.. after a few seconds it stopped.....

anyone a clue? what this is.

i have smartkey but i have it turned off ( the button below steering wheel )

 

when the keyless entry is disabled it wil not communicatie with the fob anymore will it?
so how does it now that the fob battery is low?
the car was parked and locked the fob not near it... it was just beeping...