its morse code, very common method of communication between ships 
who let you in here?

have been flashed and waved at numerous times by overtaking and oncoming MK3s
I never get this, though possibly because its always be doing the overtaking

but I've seen a few lowered mk3's around my way and none of them acknowledge me
I do get some looks when I drive through town late at night from all the cool kids, and I do enjoy the occasional blast when one of them trys it on

the ear bashing I get afterwards from the misses though is not so fun