So I'm swaying a bit from the OP's original question, but still somewhat related...
The company I work for installs magnetically-sensitive equipment in assembly plants. Before we do the installation, we do a magnetic survey of the concrete floors, and just as Andy says, we find areas with magnetic anomalies due to the aggregate or other hidden/buried utilities. We have specialized equipment to demagnetize these areas, and usually all is good. Post install, we do a final survey to ensure the floor magnetic properties are below our required threshold.
However, we did have an instance of a very confounding install where we would demagnetize the floor, all would seem to be good, and a few months later, issues would arise with too much magnetism in the floor again.
As it were, the floor in this plant was an all new, steel reinforced fiber concrete pour. So, we found out who poured the floor, and went to their plant for a visit to see their operation.
Guess how they added the steel fibers to the concrete mix? Giant electromagnets. Hmm, yeah.
So we came up with a remediation for that (company proprietary, so I can't say how), and have not had a problem at that plant for the last 20 years now.
Sorry Patrick, can't help with your problem, but it seems you have found a work around.