Infrared optics can't see through walls unless a person was able to heat the wall to well over room temperature using their own basal body temperature. It wouldn't happen.
Perhaps X-Ray optics sound a bit cliche', but X-Rays are a naturally occurring form of EMR (Electromagnetic Radiation) and are used extensively in a great deal of modern technology. It is also the lowest frequency of EMR that can pass through most solids while still reflecting enough light back to create a visible image.
Gamma optics would simply pass through everything, so there's no reflection to form an image with (while giving yourself and everyone your looking at radiation sickness). UV optics aren't practical (just bouncing black light off of your targets), and I already described the reason why infrared wouldn't work either, but I digress.
I believe that HVS should include an X-Ray optical jamming upgrade in the loadout customization so that the Phase Rifle won't be able to see you, or anyone in your proximity, through the wall.
Check out High Voltage Software's new project, "The Grinder." Coming Haloween 2011 to Wii, 360, PS3, and PC