what were The Silent Watchers?

[Chancellen the True]

Sauron seemed to have the ability to imprison spirits such as the werewolves of the first age. The odd craftsmanship of the vultures is nothing new to Numenoreans. Orthanc is probably the strangest structure in the ME of the third age. A lot of the Numenorean's architecture has an alien tilt to it. What I mean is there is no reference to where they came up with much of their design ideas. I'm a little off topic. It's an odd subject, there are other references to undefined minor spirits being used by Sauron and Melkor to inhabit various creatures (mainly wolves). Based on what types of life that are listed in all I've read, they would have to be some type of minor Maiar. One question though, what would they be doing for all those thousands of years?