Magnetization: How did it happen?

[Bangel]

Hello all. Apologies for my lengthy post, but please bear with me!

I have some questions regarding the magnetization of watches based on the following experience with my wife's Datejust.

I purchased a brand new Datejust for my wife from an AD in January this year. For the first 2 months or so, it kept remarkably accurate time, averaging about +15 seconds per month as measured by an atomic time source on the internet. By the end of the next month however, it had gained about 4.5 minutes. It continued to gain a similar amount each month for each of the subsequent 2 months, despite ensuring an adequate power reserve (full wind and consistent, adequate wrist time) and experimentation with resting positions at night.

I then took it to the local RSC. 5 days later the watch was ready for collection after being diagnosed as having been magnetized. It was demagnetized under warranty.

My questions are:

1. What kind of real world, day to day situations and/or environments would cause a watch to become magnetized?

I understand that exposure of a watch to a "strong" magnetic field can cause a watch to become magnetized, but I'm at a loss to explain how this happened to my wife's watch. What constitutes a "strong" magnetic field in this context? E.g. Sitting in front of a desk top computer? Scanning your items at the supermarket? I wouldn't have thought so, but I'm stumped and I want to avoid it happening again.

2. Is the behavior of my wife's watch consistent with magnetization?

My understanding is that a magnetized watch exhibits a variety of erratic behavior, typically running unpredictably very fast. While my wife's watch began running fast, it was for all intents and purposes, a very consistent 4.5 to 5 minutes per month.

I have tried searching for answers here on TRF and Google, but haven't been successful.

Thanks for reading and any input would be greatly appreciated.