Following several incidents of sabotage, your company hired a security firm. Among other things, this firm installed hidden cameras throughout the plant. The wall clock versions have been especially effective, as three saboteurs checked the time before committing their misdeeds. They looked directly into the camera without realizing they were providing identification of themselves.
It's your job to check the SD cards that store the videos and send the saved files to the firm. Lately, these have been coming up empty. And sabotage is increasing. What might be going on?
First, check the devices to ensure their batteries are properly charged. Do the batteries charge via a USB? Is there a time limit to the charging cycle, the exceeding of which diminishes battery life? Try replacing the battery in one device, fully charging it, and see if this device no longer has the problem.
Motion detection is a typical feature in a hidden camera. Clean the camera lens (and, if it's separate, the detector lens). Out in the plant, it's easy for these to become dirty and thus stop detecting and triggering the camera to come on. It's also possible someone figured out what's going on and simply took a permanent marker to camera lenses, eliminating any ability to record. You should also inspect the location for magnetic fields that might affect the SD card or electronics.