Out walking in the salt marshes in Essex yesterday and found some parasols and what I thought were field mushrooms. Didn't stain on picking and didn't stain on scratching the cap (sorry - no photos). Didn't even stain when I cut them up but when I started cooking them they went yellow and absolutely stunk. And that's from someone with very little sense of smell.
I've even tried scratching and cutting the discarded ones and there's no reaction at all. Just goes to show you can't be too careful.