I wonder if it's anything more than the mechanism that keeps the lid closed?

I experimented after reading your post and found that like yours, the top two corners of my MBP are magnetic.

I went to iFixIt to see if I could figure out the source of the magnetism, but I couldn't.

More: I just tested, and found that my palm rest is also magnetic.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire