The detectability values from the character class specific to stances are these by default:
Code:
<parameter class="detectability_prone" value="0.6" />
<parameter class="detectability_wounded" value="0.6" />
<parameter class="detectability_crouching" value="0.72" />
<parameter class="detectability_crouch_moving" value="0.8" />
<parameter class="detectability_walking" value="0.88" />
<parameter class="detectability_still" value="1.0" />
<parameter class="detectability_running" value="1.0" />
Why standing still is more detectable than walking, beats me.
The modifier works as an offset to the value currently active in the case.
Code:
<modifier class="detectability" value="-1.0" />
Before the actual resulting value gets used, it's clamped between 0.0 and 1.0.
0.0 result detectability of a target in eyes of AI should basically make them not see the target ever.
Can't remember if the modifier was tested, it at least isn't in use in vanilla currently, but looking at the code I can't see why it wouldn't work.