I believe that it happened during the transport of the monitor from my
stepfather's house to my apartment. He just got a new Ford F-150 that
has a built in subwoofer under the rear driver's side seat. During
transport, he placed the monitor screen down on the seat. After about a
45min trip, I think the subwoofer may have lightly induced the screen.
It is only visible on an all white screen such as a MS Word document.

The degauss method I used was the degauss button on the monitor itself.