I finally got my 'lost' luggage, four days after I arrived and you won't believe how I got it. After three days, I decided to take matters into my own hands, and good thing I did.
I decided to buy a round-trip train ticket on the German ICE, their version of the 'bullet train' (built by Siemens and capable of going at least 300 KPH (186+ MPH), which I can attest to personally). It cost 226 Euros ($325) but the company should cover it for me.
Anyway, as I said before, I knew my bag was at the Dusseldorf airport (American Airlines verified that it had gotten the bag that far, where they transferred it to AirBerlin for the last leg of my flight to Stuttgart). As I've stated, for three days we got no word out of AirBerlin as to when they expected my bag to arrive.
So I got on the train just before 8:00am this morning for the 250+ mile trip (one-way). When I got there I discovered that my bag, after spending three days listed as 'unable to forward' (there was nothing wrong with my bag, it had the proper baggage tag with the correct routing, etc.) it was declared as 'abandoned' and had just been moved to a group which would have been moved later in the day to people who would have tried to find out who owned the bag (I had TWO of my luggage tags, with my office address, on the bag) and then ship it, God knows when, to whatever address they come up with, which would have been my office in SoCal. However, I got there just before noon and the bag had only been moved off to the side but it had not yet been picked up, so I was damn lucky. I spent less than 30 minutes at the airport before I was back on the train heading back to Stuttgart (they run the trains right into the airports in Europe). So now I'm back at my hotel with clean cloths and business attire for the remainder of my two week assignment here.
Thank God I didn't wait another day, I might have never found my bag and I would have had to buy a whole new wardrobe, which would have probably been a heck of a lot more than the cost of the train ticket.