Ironically they're targeting the wrong end of the person. The sewers would be a great place to collect DNA and narrow down a manhunt in a large metropolitan area.
> CROWLEY: Right. Unfortunately, Allison's language had to be a little bit cryptic. They couldn't confirm exactly what they had done, but he writes that in some of the option they consider were testing. It's not pleasant testing the sewage in the town for genetic markers, and then, another was this UAV. Actually, it wasn't an eagle size UAV. It's much smaller.
[1]http://www.washington.edu/news/2014/01/15/dna-detectives-abl...