Beep in passengers side

Does anyone have any idea what the faint beeping is somewhere in passengers side when I turn ignition on, it stops when the car is running.
Then beeps again when turned off I thought it may be the key but it doesn't have a battery in it

Car is a freelander 1 2002 model
Any help or ideas much appreciated

Does it have an aftermarket stereo? Mine beeps a few times when turning the ignition off.

Is there a passenger airbag? If so, is there a disable switch? Could be the car letting you know the airbag has been disabled.

Roadrunner caught in the passenger wheel-arch?

Nope none of the above!!
To my embarrassment there is a inverter under the passengers seat and it seems that when someone was sat in the back has accidentally switched on so when the ignition is switched on or off it beeps only took me most of the day to sus it out!!
Thanks for all the help though