I've been thinking a bit on this, and here's the things that come to mind.
The only weapons that produce a bright "ker-ching" sound on direct hits (assuming also full-charge if available) are blaster weapons, charger weapons (minus the bamboozler) and for some odd reason, the sloshing machine. From those weapons, not all of them have been created equal, or in other words, not all directs end in a splat. The weapons excluded from one-shot kills on a direct are the sloshing machine, rapid blaster (and rapid blaster pro), the clash blaster and arguably the bambi, but that one doesn't produce the sound to begin with.
Needless to say, if a player gets a direct hit from any of these, he should be either splatted or visibly covered in enemy ink to a certain extent. Not taking ANY damage is impossible, unless...
He's been protected by ink armor, which, when he gets directed by multiple weapons simultaneously, could potentially negate all the damage due to some natural lag plus the invincibility frames past the armor breaking (which again, produces a bright shattering shound).
Though there's one more distinctive sound to look out for, which is a fully charged shot from a charger-type weapon piercing through ink armor, as seen here:
It produces a "thump" kinda sound, with the ink armor shattering sound right after. Once any hit goes over the 100 damage threshold, the user takes any damage that surpassed the 100 damage...so if the user had >20 damage, casts ink-armor and gets directed by a fully charger E-Liter shot, he's a goner, lol.
Now idk how this helped any, to me the entire scenario sounds like a massive amount of lag or invincibility frames timed perfectly with low-fire-rate weapons, as any high-fire-rate weapon tends to melt ink armor away like nothing. If you've ever tried to directly approach an N-Zap with an ink-armored Dynamo, you know exactly what I'm talking about. (Hint: Don't try it.)
Oh, and something else I forgot to mention, which may be somewhat related to the probable lag-squissue: Sometimes you might get a splat using a Blaster or charger (you got the "ker-ching!" sound) but didn't get the "[player] was splatted" notice because a teammate splatted the same enemy at the same time. With this in mind, some people just walking out of multiple shots unharmed seems more likely, if very frustrating.