You may find rats littered across the streets here and there (especially if you use Sense). Consider these snazzy blood donuts and you will want to snack on them often to refill your blood stock. There is no penalty for feasting upon them other than hearing Reid comment EVERY SINGLE TIME. Just snack on at least ten to get this trophy. You may find these in small alcoves or dead-ends out and about, but you won't be able to find them littering the occupied areas of the districts (as it's unsightly to be seen feasting on vermin in front of the citizens).