Ditto will appear as another Pokemon (Rattata, Pidgey, etc) until it is captured, including in the Sightings feature.

But if there is also a real Pidgey nearby, will the Sightings show multiple instances of Pidgey? That is, if two Pidgeys were to appear in the Sightings, we could reasonably infer that a Ditto masquerading as a Pidgey was nearby.