I am afraid that I cannot agree with Jet Ranger. As I said, it is likely to depend upon which country in Europe, but, for example, you could not do "police missions" in the UK without either having an AOC or a PAOC. The rules associated with those operators' certificates require the use of twin engine helicopters to fly at night (whatever flight rules are adopted).
I am assuming that this "police mission" involves carrying a police officer or police force employee.
In the UK it is illegal to fly in the service of the police without a PAOC, according to our Air Navigation Order.
I would suggest that there are also other practical limitations. Why would the police approach you to do some flying for them? They have existing aviation facilities. Even if they do not have availability, I imagine your helicopter will have none of the special equipment they would need to use.