From a hundred feet, or even several hundred feet how big does a motorcycle look? I'm sorry, but it's not hard to miss seeing a motorcycle. .
Yup.
People look right through us all the time.
Human sight is predator vision and, as such, primarily detects motion and is why we get 'tunnel vision' and can look right at something and not see it.
And that is why 'loud pipes' ain't near as good as shiny, sparkly, blinky, flashing things for bike safety. I refuse to look like a freaking Disco ball so, I move around a lot. I stretch my arms, legs, swerve, lane change, will flash my hi beams if I think someone who is trying to kill me may see an opportunity, just try and be 'motion' visible.
These guys were probably just cruising along, nice and steady as she goes and a virtual black hole to a quick glance, as you suggest.