Just when I posted this I came across an article. It said that police is doing investigation in the Hells Angels department. The parking place was just nearby the Amsterdam Hells Angels HQ. That means that the initial surveillance wasn't on Hillis (!) but on Donny Groen. The used parking lot was a known location to investigators, because many criminals did some talking/dealing there.
Some news on the Hillis part again.
The criminals who shot Hillis knew that Hillis had an appointment with Groen that day and time/place. How did they knew it? Police accidentally leaked a secret record about Hillis whereabouts and intentions for the coming week.
By a Dutch magazine it was pointed out that Hillis was working with the Police at the day of his death. He wore a transmitter to send the conversation to the police.
