I assume you mean ferromagnetic, meaning that a magnet will be attracted to it, but for here, we'll call that "magnetic". Steel becomes non-magnetic if:
1) You add enough alloying elements like chrome to make it more corrosion resistant, and this changes the crystal structure of the steel. Note, there are still alloys of "stainless" that are magnetic, those are martensitic stainless if heat-hardenable for things like knives and tools. If it says "austenic" stainless then it is typical non-magnetic, and gets its strength through other means than heat treating.
2) You heat the magnetic steel hot enough to where the crystal structure changes, and it is non-magnetic, and that is typically the clue of reaching the temp needed to harden the steel, and then it is quenched by liquid or gas, and once no longer that hot, it is magnetic again.

Very few bike frames are stainless steel. Some customs, and some first model Dahons are claimed by some to be stainless steel, but it may just be chrome or nickel plating, I don't know. Typical "Hi(gh)-Ten(sile)" steel and 4130 chrome moly steel used on steel bike frames, is quite ferromagnetic. Ships in WWII were "degaussed" in shipyards, passing through huge coils, to remove magnetic charge, which would make them more easily detected by magnetic anomoly equipment, and it messes with magnetic compasses. This degaussing would need to be done periodically.