You could keep putting human footprints around their camp when they're out foraging. And have stuff missing, yet there's no one around.
You could get some guy in a Bigfoot outfit and have the schmo keep seeing him.\\
Or you could just play little mind games like move his camp or move equipment and let him think he's losing his mind.
The possibilities are endless. Ah, if only I had a production company full of like minded evil people.