Don't throw their shit around, just move it slightly. They are watching tv, reach for cup of tea... Its now 3 inches to the left. They go take a shower, shampoo is now at front of shower instead of back. Heading to work, their car seat is all the way forward and the back is angled forward. Randomly switch the direction of rotation of their bedroom ceiling fan.

No one will believe them. They will know something is going on. They tell friends and family but are told they are imagining it. They keep insisting it's real.

Do something too big and it ruins the psychological warfare. No one can deny their car was flipped over and thrown into the car next to. No one can deny the TV was ripped off the wall and smashed into the family dog. But if their spice cabinet just was re-arranged three times a week...