Ryuki's danger sense
Warning--very extremely spoilery. Seriously. I mean it. You so will not be able to unread this!!!
Looking at the current In Nomine mailing list, it looks like Yves' people have a particular set of advantages--mental note to reread my Yves pdf. That doesn't apply here, but still, rereading time.
LOL I hope none of my players have this on e-mail feed...don't look down!!
I already had a good sense of how Ryuki's sense works when I had him flinch at Ash and Zahira--he does have an innate sense that lets him detect celestials. In his backstory he got ambushed and nearly killed by a room full of demons, so that sense can be fooled, bypassed, or ignored.
So it works like this: what he's picking up on is total Forces. He can't spot a single angel--the number isn't high enough. But if there are 18 Essence where there should only be ten, he can spot that difference in a crisis situation. This is very simply a survival instinct. Why didn't it help in the ambush? Maybe it took time to develop, something honed through meditative/sorcerous practices. That would suggest that his mother does not have it--she can actually see celestials, so what's the point in sensing them? He'd know that's a hole too--if they show up in a vessel she will only probably know it.
But back to the barfight. He probably just underestimated them, more likely didn't believe they would work together. Demons generally don't, selfishness keeping them divided. It's how he can operate in Los Angeles, and what he does there. Keeping them divided makes it much easier for him to sell them gossip, which he trades for Essence. Keeping them divided means even those who know about his friendship with an angel won't be able to do anything effective about it. Now when they figure out he has a friendship with a few angels, that could be enough common enemy...
It's a survival instinct, so it's enhanced by the perception of danger. At SFO if he hadn't been riled up he wouldn't have spotted Ash and Zahira, but he did because Shen got his adrenaline up. So to be a little crunchier:
Two PC-level celestials he can only spot under perceived dangerous circumstances, and only if they're standing close to each other and to him. A base number too low brought into range by bonuses. He might perceive this like a crowd-sense, but since the overarching metaphor is music I think I'll go with an audio function. Excess essence sounds like a hum, like the noise of a fluorescent light, a refrigerator etc. Usually he tunes it out, but it's almost always there. That's why he finds such sanity in Hawaii--that noise generally isn't there.
Normal human is 5 Forces, normal celestial is 9. Difference of eight is only perceptible under ideal conditions 18-to-10 is too small a number without bonuses for proximity and survival. 18/10 ...target number should then be higher than 18. 5 Essence is silence to him as is 6, no matter how many people are there producing it--so we're definitely looking at a proportion. 5 then isn't silence--it's background noise. He's hearing proportion, so a celestial will sound...thicker. It would take (arbitrarily) four celestials grouped together for him to spot them under ordinary conditions. Yet despite a room full of celestials (Dragon's Lair) he still had to rely on deduction to know which ones were angels. Too much noise jamming the system. It's a blunt sense even after years of refinement.
Valefor pulled him out of a dead sleep but that was an active Disturbance, not mere existence. Ditto much of that tether--he knows there's something back there because it's noisy, but he can tell it's angelic now. That came with experience--he probably needed Corat's explanation to realize that those were all angel noises back there and that he could rest easy. As a result he has a much better handle on discerning angelic noises from infernal ones. He'd have a dim sense of band with demons, having heard a few, and can probably place any noise to within three bands. This comes simply with practice; he can't peg choir to sound just yet.
Totally surreal world he lives in--no wonder it's taken so much intervention just to keep him sane.