Thanks for this Chris.
I will search the vehicle, I doubt I'll find anything as I have had Alphie for quite a while, and been through all the storage cubbies before; although not searching for a hidden key.
If there was a hidden fob, surely the system would prevent the vehicle being locked which I can always do.
I always use the smart fob for locks and the immobiliser for starting.
That said, maybe my van 'thinks' a smart fob is in the vehicle when it is actually in my hand less than a metre away.
What can be annoying, is when I step out the vehicle with the intention of opening another door (perhaps for my dog to get out) the car locks as I move much less than a metre away from the driver's door.