For me the only way to make psi power "realist" should be to give a range, independant of line of sight, so for exemple 20 square radius. And alien, or human with psi amp, can only psi-strike something within this range. So even if another soldier have spoted something, if it is too far, psi attack can't be possible.
But, in this range, any aliens (or human) should be spoted, even if it is hide and not visible with eyes. This is in my mind how should work something like mental power.