Here are a couple PDFs for your consideration on airborne magnetometer surveys:
https://www.geosci-instrum-method-da...5-181-2016.pdf
https://www.geosoft.com/media/upload...vey_Reeves.pdf
If there are enough other ferrous minerals in the area, that may help mask the location, but a solid steel door should still show if you take more passes closer together.