Science speculates on what happened to the magnetic field of Mars, whether it was catastrophic or slow enough that no one noticed. Did a giant asteroid or two knock the heat flow in the middle out of commission? We know that it had a magnetic field once, and that without it it has ever since been systematically stripped of atmosphere and water and tiny televisions and shades of green, we just can't be sure what made it up and leave one day. I imagine that being a magnetic field is not particularly interesting, always guiding everything else and only staying put, and so I find it likely that it used the asteroids as an escape and is still out there somewhere, touring the universe.
The better story lies somewhere in what would happen if it came back.
The better story lies somewhere in what would happen if it came back.