Watch this from 1.32 and you'll see how they are able to spot them so quickly. A quick message to the stewards in the away end and an ID check and you're spotted.
Cameras in Dutch stadiums are no joke.
If you threw it into the Ajax end, I have no sympathy. Dutch football has this problem every week and they are postponing games every time a beer is thrown onto the pitch with 4 games AT THE SAME TIME being stopped a few months ago...