How did someone get a key to our car?!?!

[Scott Sutherland]

My wife's 2001 Audi A4 Quatro was stolen about a month ago, and it 
was just recovered.  Surprisingly,  guy the police arrested in the 
car actually had a key!  It wasn't one of ours -- we still have the 
three originals.  This  was a straight key (not one that flips out 
from a key fob).  It has a metal blank and a square-ish plastic head 
that has an imprint of something that looks like a wrench, just below 
the Audi logo.  On one side of the blank it has "AKS", and on the 
other there's a small "w" near the head and "NAA" near the top of the 
groove.  It appears to be a valet key, since it won't open the trunk. 
We're in Seattle, Washington, USA, if it matters.

Does anyone know what kind of key this is or how the thief might have 
obtained it?  We're pretty upset that a car that was supposed to be 
so tough to steal was so easy, and any help/insight will be much 
appreciated.

Thanks in advance,
Scott