If you live in DC, call: 1-800-535-7252
If you live elsewhere, I'd suggest 911, but I bet you could find a number like the one above.
** I think that you don't have to wait till they show up, just give the address (look at building numbers) and say "she's lying under the Mazi Pizza store" or something. The number I call just asks for a brief description, "blue jeans, brown jacket" - nothing long. And there doesn't seem to be a mile of people lying so there's no chance of a "mix-up" (stupid joke).
Please stop walking by, and call. Speed dial.