What a hotel card’s stripe actually stores
A magnetic-stripe room card carries its information as tiny regions magnetized in different directions. When you swipe it, the lock’s reader senses the changes between those regions and turns them back into data. A strong enough magnetic field can rearrange that pattern, leaving the reader unable to recognize the card.
Hotel cards are often encoded on relatively easy-to-write magnetic material. That makes them convenient to issue and reuse, but less resistant to powerful magnets than some payment-card stripes.
The real pocket hazard is a magnet
Ordinary keys, coins, and other unmagnetized objects do not erase a stripe just by touching it. The risk comes from exposure to a magnetic field strong enough to disturb the stored pattern—for example, from a magnetic clasp or another powerful magnet pressed close to the card. Scratches, bending, or dirt can also interfere with a swipe, even though they do not demagnetize the data.
A phone beside a card is not automatically a problem; the relevant question is whether a strong magnet is close to the stripe. Magnetic phone accessories and closures are more plausible culprits than the phone’s mere presence.
Why a failed card is usually easy to fix
Hotels can generally encode a replacement card at the front desk. If a card stops working, it may have lost its magnetic pattern, but it could also be damaged or simply no longer be authorized by the lock system. Keeping it away from strong magnets is sensible; treating every shared pocket as a threat is not.
