An IT staff member found a Palm Pilot stuck to the underside of a server. It was held there with double sided tape and had a cord running out of the device. The engineer followed the cord, found out it was plugged into the core network switch in the data center. It was packet sniffing network traffic that was being routed within the entire company!
After the staff confronted the vendor that had put it there, and the vendor admitted they were sniffing packets, the vendor was fired and asked to never return. Sadly, you can’t make these stories up.
Moral of the story: Keep a watchful eye out for all data center physical activity.