Minerals show along the bottom of the scanner closest to you. The stuff that shows at the top of the scanner indicates a POI. If you see a reading that covers top to bottom its normally a ship (crashed)
Sometimes you will see random short 'shadows' (top to bottom) appear on the scanner. I think these are POI at extreme range.
If you stay around 2km-5km above the planet your scanner will show POI as large blue circles.
You can often see the POI from around 2km in height anyway, although if they are canisters it can get tricky to spot.
Hope that helps
Thank you for the rundown/ confirmation - it confirms what I suspected. Have some +rep for taking the time to help me out.