The first time it happened, the car was in my garage overnight, as always. I would have heard the alarm if it had of gone off.
The second time it happened - the 5 beeps - I was in traffic on the M11.
I was stationary for quite a while so I turned the engine off and sat in the car for a while.
I didnt lock the car obviously, as I was still in it!
About 5 mins later, when the traffic started to move I started the car up, and then got the 5 beeps.
No warning lights etc on either occasion. Nothing else to show anything is wrong. And as stated above, both times the car still starts and drives normally.