A couple points to add as to what I do when I encounter these situations.
1. I slow down to cool-down lap speed and constantly scan the surface. In other words...WAY SLOW!!!
2. I never assume that one visual result is confirmation of the flags. I am overly cautious the entire way back to the pits assuming that there is something EVERY FOOT OF TRACK the rest of the way in.
3. Lastly, and most important when you go back out......do NOT get back up to speed for a couple laps. Use that time to scan the surface and feel the grip level checking for any missed remnants of fluid, etc.. Also watch the body language of the cars around you as they will give you clues as well.
As far as the videos, I will refrain from comment
