With the mk5 if you open the boot only with the middle button on the fob it will auto lock as soon as you close it.
As Topher says, never leave your keys in the boot.
That does sound a bit scary, at least I didn't do that. (Not impossible I might have, though.)
If it's set to single-click lock/unlock, will it still do that? I assume it might, if you just used the centre button. However, if it's set for single-click, and you always use the door button to open everything even if it's the boot you actually want, will the doors auto-lock on you?
I suppose I should go try it, but it's chilly out there!
Rolfe.