When a weapon with nightstalker sights is picked up, on first use it has four different modes: visual, night vision, greyscale white hot thermal, red white hot thermal.
Once the weapon has cycled through these modes, on all following cycles the third mode will have changed to red thermal black hot (from greyscale white hot thermal). If the weapon has not been picked up off the ground, the correct red black hot thermal mode will be displayed.
This pattern repeats if the weapon is dropped then picked up again.
Weapon sights should work consistently in all circumstances.