Heh heh.
I did a smilar thing a few years ago. Was marking arrows in the ground with flour to mark courses for my running group near the GG Bridge. I didnt think much of it until the officials at the bridge district confronted me one day near the toll plaza, while i was putting the final touches on an arrow, and took down all my information.
If you rub the flour into the concrete hard enough, it stays there for serveral weeks--not even the rains will wash it away!
Glad the left coast liberals are less uptight about these kinds of things, or I'd be in Guantanemo by now.