Interesting to see a "new conspiracy theory" taking shape: or an old one being "verified".
According to Der Spiegel the police were only able to check the drivers cabin (where the ID was found), because they first had to have the search dogs in there to smell out the person last driving the van and thus identify him. They therefore got down to cleaning the place once this had been done and admittedly valuable time had been "lost". Der Spiegel quoted police as saying that the reason they believe the ID (it seemed) was purposely left there was to drive and feed more hatred into those folks who are convinced that the people fleeing their countries from wars should be sent back to where they came from. A psychological strategy that IS uses to terrorise an already insecure population... increasing hate and fear between Muslims and Christians.
Will try finding the link and posting it for those who are interested in reading the german version.