Wrinkled fieldcaps aren’t particularly inconspicuous, but the first ever record of the species was as recent as 2003, in the Netherlands. Then the following year in Britain. Now, they’re quite common in western Europe and England, though still occasional in Scotland and Wales.
For once, the explanation may not be climate change, but other aspects of human activity. They grow, pretty much uniquely, on wood chip mulch. A recent spreading of wood chips is quite likely to be hosting a troop of them, especially if it’s in a thickish layer.
So despite the name, one place they aren’t found is in open fields. Sometimes the caps are indeed wrinkled, in furrows spreading down from the peak to rim. At other times the caps are smooth.