Do you have another key that works? If your spare works, then that's a good sign.
Take the key you're having trouble with and leave it in the ignition for a few minutes. I think I remember reading that this should reset things... but I might be completely mistaken. Doesn't hurt to try though.
So I compared both key fobs. When I normally press the button on the bad key fob, it doesn't light up. When I hold it down for a little longer there is a flash, but it is delayed where as in the good fob as soon as I press the button the light flashes.
All I know is the fob has lock, unlock, trunk and panic on it. Don't know much about DAS3 etc.
The dealer is your only choice for a new key.
Be prepared for a shock...
I just overcame the sticker shock. OP, my key fob remote functions didn't *always* seem to register, and it got to the point where I HAD to be meters away for it to function. I also noticed that once in a great while I would have to reinsert it into the ignition as it wouldn't turn. This was about a year before it just quit registering altogether (remote and ignition lock) leaving my car parked in the driveway and the sunroof stuck open. The tow was complementary, and the $450 dollars it cost me was completely in vain once the service department pulled my car up. The negligent tow truck operator Mercedes sent to my home put an $1,100 dollar dent just underneath the driver side tail light. Currently battling it out with customer care going on two months now, I'm ready to either start a claim with my insurance or just sue. I honestly don't care at this point, I just want it fixed!
Your in for the long haul, or the long stall. Just waiting for you to give up.
They will say prove it wasn't there before. Do you have recent photos?
A lawyer can write a demand letter saying if they don't respond in 7 days then you will take action. The word action can mean you will jump up and down, wave your arms, hang a moon or whatever. Depends how smart in the legal game the other person is.