SS2: Motion sensor

[664a4a82323a9paganinio]

Motion sensor (PSI Tier 3): Shows the location of all nearby creatures for 30 seconds per PSI.

How reliable is this PSI ability? I often see red dots behind the walls, how could there be any creatures? There are also some red dots showing up in empty rooms. Also, sometimes there aren't any red dots when I can see a creature with my own eyes.