The Mirror - S3-E6
Visible crew/equipment: Clemente walks over to the portrait of the former ruler, and throws his glass. Just as he completes the action, the camera looks at the portrait. There is a movement of what appears to be a device on the right side of the shot firing the glass.
Hocus-Pocus and Frisby - S3-E30
Visible crew/equipment: When Frisby tells the two travellers that they're looking at the grandad of the American automobile, we can see the reflection of a crewmember wearing a suit, possibly the director, who stands up from a directors chair (also seen in a following shot). Then as Frisby tries to locate the gas tank, the shadow of the moving boom mic is visible on the store's doors. (00:06:20)
Hocus-Pocus and Frisby - S3-E30
Visible crew/equipment: After the two travellers have paid for their gas, when Frisby advises them to check in someplace because of the clouds, the reflection of the moving boom pole and mic can be seen above the word "shop" on the store window. (00:08:00)
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.