Maybe be arbiter itself was cloaked by disabling (not sure if it's possible) or just there for a short time for to create the cloaking effect and removed before you noticed it?

No there was no arbiter placed or created within the map at all. The computer could cloak one unit in maybe 100 tries.

100 DCs. Hypers cause the game to run through triggers once every frame.

Nope. With hypers it's 2 frames per trigger loop, so 100 frames = 50 DCs.