The Night of the Trolls by Keith Laumer

Flat on the wet ground, under the wind-whipped branches of an ornamental cedar, I blinked the fine misty rain from my eyes, waiting for the half-hearted alarm behind me to die down.

There were a few shouts, some sounds of searching among the shrubbery. It was a bad night to be chasing imaginary intruders in the Baronial grounds. In five minutes, all was quiet again.