See I don't believe you, I already heard about this yesterday and spent about an hour testing it.

No random witness pops out of nowhere, it only happens if there is a witness there, the ISSUE is that your console might not be rendering a witness that IS THERE but you just can't see them until you get a bit closer making it SEEM like they came out of nowhere.

I shot multiple people, and at one point I even carried a body down to a road, just to see if I'd get the mysterious "witness" pop out of nowhere, a single horse cart came and when he was about 50 feet from the body, did a 180 and the witness dialogue popped up.

I even took a guy hogtied out into the middle of nowhere, made 100% sure no witnesses, shot him, and....

NOTHING....

Until I took his body near a road and the first passerby flipped out

(keep in mind I can't even tame a horse so some of this may be bugs still..)