Quite possibly due to either loss (retentivity decay) or the effect of the earth's magnetic field. If the location has EM fields from large induction motors or even AC transmission lines a few hundred meters away the effect can be augmented over time. I've come across tape reels that had this phenomenon and the levels were a good 6dB+ lower where the flux lines crossed. It's fixable in the digital domain nonetheless but interesting. Lots of those tapes had print through as well but now I'm getting a little too OT.
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