I've been taught that a compass should never be stored near a strong (electro)magnetic field because it could permanently affect the compass needle accuracy, maybe even reverse its polarity. It's a pretty common bit of advice that I've come across in various manuals and literature. Silva website also has the following to say about the matter:

"Please be aware that a reverse polarity is caused by exposing your compass to articles with iron content (something as simple as being placed next to a pair of scissors or a knife for a length of time, microwaves, high tension wires, etc.)."
http://www.silvacompass.com/frequently-asked-questions/#3

And this article:
http://www.grough.co.uk/magazine/2013/04...needle-walkers#