What does it actually do?

At least in the beta, a creature with Unnatural Senses can see or hear out to the range listed in the parenthesis, even if they have no eyes or ears. For example, under the definition of Blind, it excludes creatures with Unnatural Senses from the penalties. The psychic power Warp Perception (again, in the beta) does nothing but grant the Unnatural Senses ability, so it seems likely to do more than just enhance vision.

Some creatures with the trait feel the presence of their enemies via tremors, but there's really nothing to suggest the trait grants anything more than an approximation of normal vision. A psyker with the power essentially can't be blinded, but if it has other intended uses (which is likely) it's not immediately apparent.

The closest thing to an actual description of the trait's effect comes under the Sentinel Array, which grants the user Unnatural Senses, and says "can see in all directions, and can detect targets through... obstructions." But it could be argued this more generous ability is unique to the Array.