The Color Purple

Here's a mystery for any chemists or cleaning geniuses out there. Our sink is made of some kind of metal, probably stainless steel. Every now and then a purple substance appears in the sink, looking like it was dropped in roughly the same spot each time. Both substance and source are mysteries.

We've looked around the sink. We've looked on and around the window sill. We've looked at the faucet. We've looked under the sink. We've looked in and under the cabinets over the sink. We've dismantled the flourescent light fixture under the cabinets. There are no ink pens, paint cans, markers, crayons, spray paint cans, rabid squids, or punk elves with too much hair dye hiding anywhere that we can see.

The only pattern I can see to the occurence is that it happens mostly during cool weather. We had this purple enigma show up almost daily when we first moved in during February, then it tapered off, and we've only recently seen it return.

Any ideas out there? I've even searched for "metal sink purple" on Google and, while I found an amazing newsgroup post by some goth child wanting purple hair and stainless steel fingernail implants, I found nothing giving any hint as to what might be causing our mysterious purple sink.