I tested one mine shaft, and near the bottom it started reading very high (20-40) still standing on the same square. I think this may be a result of the equation used by the OV scanner - where value = 1000 * ore-values / # blocks below player (in 9x9 area with player in the middle)
The miner had stopped at the first piece of bedrock it hit, and I found some diamond beside the bedrock on a lower level beside it, however that was the only shaft so far that I've been able to then get a zero from the OV scanner afterward.
Every other mineshaft I've tried, I get a reading >0, so my conclusion is that there must be some ore stuck below bedrock which the OV scanner picks up, but which is inaccessible (damn you honeycomb lattice bedrock and your inability to be mined!)
My Solution: mine elsewhere and stop being so anal retentive about getting 'every last piece of ore' - there's lots of fish (and ore/clay/squid) in the sea.