One day I was using the Prizm III and on a sweep in I hit a target. In one direction I got the (good) "Blink-Blink" sound, but when I sweeped in the other direction I got the (bad - iron/junk) "Bonk". It showed at about 8" so I didn't bother digging it. Was it really junk? Has anyone else had a sweep reation like this?