Chapter Master, how is it that an 8 foot tall superhuman in shiny white armor, wielding a heavy weapon...

Chapter Master, how is it that an 8 foot tall superhuman in shiny white armor, wielding a heavy weapon, and driving a noisy jetbike is stealthy enough to scout?

the rules of the game aren't something to be taken literally

It's the Grim Darkness of the Far Future (TM), I ain't gotta explain shit.

They're just that good.

They can't detect you if they're dead.

Same way that half-ton super-soldiers in full metal-and-concrete battle-plate with a mini nuclear reactor on their back is described as moving literally without a single sound in the fluff.

Same reason modern militaries use big noisy buggies/jeeps to do recon work, you dont need to stalk around, just out-maneuver your opponent, gather information and fuck off back.

But what about actual scouting? When they want to remain fucking HIDDEN because there's an entire Waaagh! or some shit?

Yea this. Scout tanks are probably at least as noisy as those bikes ( I would say definitely but motorbikes can really shriek).

It doesn't matter if you're a mile away. They scout round the flank a report artillery coordinates from the safety of their armoured box then if need be race back to safety.

Scouts aren't infiltrators they just operate ahead of a main force.

Please refer to