Had something similar. I got a school calendar and figured out the days they were off and then did survelance the night before.
You could also try to find out where they are buying these things and ask the clerk there to get a license plate number.
It worked for us.
A camera would be very wise.
Have you called the Gestapo?
Great advice---and I actually tried that. I live in northern MA, and around here, you can only buy fireworks legally in NH. So I called up all the fireworks purveyors in southern NH and of course I got "we don't sell those," "I don't know what you're talking about," etc.