Hi there. I'm far from being an expert, but my assessment is this.... The Infinium, being a pulse (PI) detector, by its very nature is designed to see through hot rocks and ground mineralization. I apologize for being unfamiliar with the terminology used in this context, but according to standard definitions the rock you described above is not magnetic. If it were a magnetic rock, it would be the one picking up a piece of steel or iron. Here's a link to a picture of a magnetic rock:
http://www.zentropolis.com/logs_2003_12_03.htm (you may have to cut and paste this in your browser)
Rather, the rock has concentrated iron within it which makes it able to be picked up by a magnet. Most of the hot rocks that I have found are larger and do not have enough concentrated iron within them to be able to be picked up or attracted by a magnet. I found some small ones last weekend that were as you described above - they had enough concentrated iron in them to allow them to stick to the rare earth magnet I have stuck to my pick. In the case of a PI detector like the Infinium, it is more of a PI issue than a ground balancing issue. The other detectors mentioned in this thread appear to be VLF detectors. And for those detectors, the ones that auto ground balance will balance out ground mineralization which would reduce their ability to recognize rocks with iron content. They will completely ignore rocks with low iron content but may still be able pick up rocks with high iron content. Most auto-ground-balancing detectors (like my MXT) have a limit as to the extent of the mineralization that they cancel - this is designed to ensure they don't completely balance out and ignore real targets. So, based on the amount of iron in the rock, an auto GB detector may or may not be able to pick it up. It sounds like the meteorite that you tested above had a level of iron content such that the GMT completely balanced it out, and the Infinium saw right through it, exactly as it is designed to do.