Big drones like the Reaper are conspicuous because of their engine noise; the Predator is described as making a noise like a lawnmower. Smaller tactical drones like the Raven, Desert Hawk and Maveric with electric propellers are inaudible above a few hundred feet, and the only sign of their presence is from their silhouette.
A technique called counter-illumination was explored during WWII to make an aircrafts outline merge with the sky above by means of careful lighting. This was only partly successful, but with new technology — specifically commercial e-ink panels — it is now possible to cloak a small drone with adaptive camouflage.
The technology is being developed by Plextek in the UK, and I describe it in more detail this week in this article in Popular Mechanics.