Mysterious pink marks on polyutherene mat?

[ABDefender]

These are showing up overnight, we think. And we're totally baffled.

We've ruled out pencils, pens, markers, make-up, drinks, food, chemicals, toys. The mat is used by kids daily.

They won't come off with wipes, toothpaste. Afraid to use anything strong in case it takes the colour from the mat with it.

Any ideas would be so very appreciated.

What is the mat used for and how have you ruled out the above substances?

Would something slightly abrasive like a toothbrush or sponge scourer damage the surface? Bio washing powder/ bar keepers friend/ cif cream are cleaners that often remove stains successfully. Sunlight (, although more difficult in winter) or lemon juice perhaps.

Kids playing - and they don't have anything that matches the colour. Tested the pencils, doesn't transfer and wrong colour. Nothing plastic that colour. Don't have make-up.

All of those sound like things worth a try. Just wish we could figure out why they're showing up literally overnight.

If not a toy, maybe an item of clothing like a button/ hair slide/ shoe sole rubbing?