Bend Time Level 2 claims that aall kills count as being undetected while time is frozen, but this is not always the case.

In the level 'The Loyalists' there are three guards and two tallboys pinning down Piero and Sokolov. The two at the back I froze time when just behind them, stabbed the first guard while crouched, and the second guard had the alert come up, so when I stabbed him he did not disintegrate as he would have, had it been considered a stealth kill.

I tried this again several times and realised that I was stabbing the guard who was standing ever so slightly in front of the other one, so it was in his line of sight.

Ordinarily this would obviously count somebody witnessing it, but as I had frozen time, this should still have counted as stealth. This has also happened in other situations, sometimes it's even the first person I kill while time is frozen that counts as a non-stealth kill.

Was there a music box Overseer nearby that could be disrupting the magic field?

Also, towards the end of the game, some enemies resist Bend Time.

Is it possible that the detection happened right before you used bend time?