Doors locking themselves

I have just noticed a strange thing with my 2010 Chevrolet Aveo sedan. If I unlock my doors with the keyless entry and wait about 30 seconds the car horn sounds and the doors lock by themselves? Is this expected behavior or did I inadvertently turn on theft deterrent or something? I read through the manual but I'm still not sure what it is doing exactly?

Thanks :)

It is supposed to do that. Its basically for if the unlock button gets pressed by accident when the fob is in your pocket. My moms Saturn vue does the same thing, but it doesn't do it till about one or two minutes later

interesting, thanks for the info