From the FAASafety Team:

Your Aircraft Systems May be Vulnerable
Notice Number: NOTC4031

• Many low-wing airplanes, including some regional airliners, as well as helicopters, are susceptible to magnetic disturbances caused by buried- and surface-objects containing or made of steel
• Directional heading errors have contributed to loss of separation between aircraft during initial departure and subsequently caused the issuance of pilot deviations
• Such errors have been documented to be up to 40 degrees
• Errors in heading may be indicated on one or both installed slaved directional indicators (HSI, EHSI, PND, etc.) which MAY or MAY NOT be detectable and alerted to the pilot by a comparator monitor, if installed
• Heading error indications vary in magnitude and may be observable in the same or different directions, depending upon flux field density

For more information and what to do click on the link below:
https://www.faasafety.gov/files/notices/2012/Aug/CAUTION_TLP_031612_Rev10.pdf

USMCFLYR

Note - I've seen this every once-in-awhile.
I think Mountain Home, AR was the last time I can remember.