One of the tactics that the British used against the IRA was to open a laundromat. No, really.
The plan was simple: Build a laundry and staff it with locals and a few of their own. The laundry would then send out “color coded” special discount tickets, to the effect of “get two loads for the price of one,” etc. The color coding was matched to specific streets and thus when someone brought in their laundry, it was easy to determine the general location from which a city map was coded.
While the laundry was indeed being washed, pressed and dry cleaned, it had one additional cycle — every garment, sheet, glove, pair of pants, was first sent through an analyzer, located in the basement, that checked for bomb-making residue. The analyzer was disguised as just another piece of the laundry equipment; good OPSEC [operational security]. Within a few weeks, multiple positives had shown up, indicating the ingredients of bomb residue, and intelligence had determined which areas of the city were involved. To narrow their target list, [the laundry] simply sent out more specific coupons [numbered] to all houses in the area, and before long they had good addresses.
Clever. (via schneier)