I need a word for "use of it is hard to recognize until it is too late/hard to determine origin of threat". I should stick to just sneaky.

I don't doubt that one can make a sneak attack with a baton (you can do it with your fist after all) but you still have to walk right up in someone's personal space to do it, and a person viewing the altercation, or filming, from several feet away can follow the chain of events and determine blame. A video of a cop sneaking a blow with a baton is hard to deny in court (or if it never makes it to court, the court of public opinion). A spray of chemical irritant mist from inside a crowd or a grenade tossed from outside a camera's frame? Who knows which police officer did it, and the attacker can be several feet away from the recipients (or in the case of the grenades and whatever you call those paintballs filled with irritants they used in Seattle during the WTO protests, several yards).