My office is a microwave

My mug of water in the office always seem to have a layer of blackish residue in it. I tried changing mugs, overturning the mug before i go home, a layer of filth still appears when i return to the office the next day.

The thing is, when i overturn the mug, i would have supposed that nothing could have gone into the mug. Yet the layer of sooty grime appears! This is kind of thing never happened to the mugs at home. And the only difference between my home and the office that i can think of, is the pressence of hundreds of rfid devices, some active, thus transmitting signals overnight.

Could the blackish layer be a layer of something else, charred by cooking in the office, like how microwave works? I have no idea.. I thought i needed a tan, but i didn't think getting microwaved was the way to do it..