A few days ago, whilst leaving the house in the 500x, an alarm went off saying a door was open, the driver's door handle happened to be stuck out as if someone was pulling it, a little wiggle made it click back into place as if it had been frozen.

Fault or a one off?

I bought a used 500x last week, and the driver's door handle has some fore-aft play, which sometimes makes it foul the fixed part where you would put the key. This has made it stick in the "open" position a couple of times. The boot release above the number plate is loose, too.
I took it back to the garage to show them the issues, and they had a technician take a look. Apparently replacement parts are needed and they will take the car in to fix it when they arrive. I will keep you posted.

There is a warning in the Owner's Manual about the unlocking & door handle pulling.

It is possible coincide the unlock halfway through its process and pull the handle and unwittingly force the internal mechanism.

I had the doorhandle/boot release problems fixed by the dealer earlier this week. All is well now, and they even washed the car while they had it. The icing on the cake was the courtesy car they gave me - a Fiat 124 Spider. Very nice, and very crafty as it made me want one!