I can so relate to this, my husband was laughing when he saw it. I do the same kinds of things. Years back I was scrubbing one spot on the living room carpet and complaining to my husband that it wasn't coming up and I didn't know what it was. He pointed to the punching bag cat toy I had bought that stuck onto the wall with a suction cup....it was casting a shadow.
Same thing in the bathroom sink, I was scrubbing a stain in the sink that wouldn't come out, it was the shadow from the PUR faucet water filter. After those things, I now look first for something that might be causing a light or dark shadow.