Last summer in DC a friend and I were engaging in getting rubbings of a few manhole covers. All was well until I looked up to find a swat-type guy dressed in all black and carrying an automatic weapon who appeared to have materialized out of thin air. He was quickly joined by a motorcycle cop, 2 police cars, and a police foot patrol backup.
After checking our creds against known terrorist lists, they politely asked if we’d move to an area with less foot traffic. Actually the swat-type guy was pretty nice the whole time, and was the last to leave, ninja style. I swear, I never saw him either coming or leaving.