I now know for certain that the feature is bugged:
- I logged off about an hour and half ago with 4 collectibles which I could no find, even having scanned the map carefully in maximum zoom, and in grid pattern: vertical, then horizontal, then once more vertical then horizontal.
- I had looked with even more care near EVERY building, where they are often hidden by the presence of buiding at the same spot (but being in front of the building).
- I had 15 minutes to spare, so I just logged in an checked again; and guess what, I just found 2 more at spots I had very definitely checked carefully earlier, and they were not visible there at that time.
So, my conclusion: The problem seem to be that the items are not always loading correctly when the map load, or when they are added; THAT is a bug in itself.
Workaround: Scan, press F5 to reload, wait for the map to be fully loaded, scan again, and so on.
It also explains why friends may look at your map and see collectibles which you did not, even if you looked at the SAME spot.