You get revealed in the moment the bomb is placed. Cast time - invisible. Spell goes active - temporarily revealed similar to if you cast a healing ward on yourself. Spell arms - invisible. Spell goes off - revealed.
This is why it is a NB stunt - cloaking upon activation prevents the temp reveal.
That's good to know. Was wondering how this casted it considering i saw him went to sneak mode then a few secs later he just jumped me with wrecking blow and detonation immediately right after.