And I have double locking enabled on mine... though the documentation does not even show it as an option. It is one of the '06 early vin models so perhaps an oversight later "fixed" by an update??

 

If you have a North American market X150 and believe that your car does, in fact, have double-locking enabled, you can test it to be sure.

Sit in your car, either seat, and give your Smartkey to a friend. With both doors closed and the windows up, have you friend press the lock button twice, until you hear the double beep. Then have your friend walk more than 30 feet away from your car.

If you can open the door, your car wasn't double-locked. That double-beep is an audible error message telling you that you already locked the car. To confirm that it's an error message, leave your door open and press the lock button on your Smartkey or the door handle. You'll get a double-beep.

Hopefully, your friend comes back with your Smartkey.

 

Or if you don’t have a “friend” you can just roll down the windows and do the same test 😉

 

Double-lock isn't a thing on US cars as the TRUE "double lock" also disables the door handle on the INSIDE of the car. Very dangerous if you have anyone inside the car when locked.

The sounder beep just verifies LOCK status in the US.

 

That's what I previously explained. Double locking prevents the doors from being opened from the inside, which is illegal in the USA.

Read my Post #3 again, and click on the link to read the explanation by NHTSA.

From the Owner's Handbook:

 

Right, so tested mine last night. Single press, doors lock. Double press mirrors fold, signal lights single flash and beeps once. Double press also causes electrics to immediately shut down.

With driver’s window rolled down, interior door release does indeed function and sets off the alarm.

 

And I just tested mine. Single press, doors lock, mirrors fold in, and amber turn signal lights (front and side) flash once. Maybe that consolidation of functions was a programming improvement for 2009. iPod connectivity became standard equipment on the 2009 models

A quick double-press has the same results plus a double-beep of the sounder. That's an error message because the first press did it all, and the second press accomplished nothing.

 

And over here in the UK on my MY10 XKR, one press does everything Stuart just said, but on the second press you can hear the deadlocks engaging. The second press also enables the interior volumetric alarm, as I discovered to my chagrin when I parked up in a car park and left it double locked with the roof down. Returning to the car I threw my shopping in - and set off the alarm...