The discovery of the device on May 5 spooked law enforcement officials, who have been on heightened alert for suspicious activities around train tracks since thein Spain in March.
Officials at the Southeastern Pennsylvania Transportation Authority said an employee told the FBI he put the device by the tracks to alert him to approaching foot traffic, "presumably his supervisors."
"It was in a perfect place if you wanted to get some sleep and be alerted if your boss is coming," SEPTA security chief James B. Jordan said of the sensor at a news conference Monday. "He knew the route his supervisor would be walking across the yard."
The transit agency did not immediately identify the employee. It said he worked full-time on an overnight shift for SEPTA and part-time for a security alarm company. Hearing of the alarm the discovery of the device had caused, he turned himself in to authorities.
The device, found by a conductor in a yard near Philadelphia's massive 30th Street Station, was, and ostensibly could have triggered a signal or alarm when it detected motion, SEPTA spokesman Richard Maloney said.
No receiver was found, he said.
FBI spokeswoman Jerri Williams said the case remained under investigation Monday, with agents trying to determine whether the worker broke any laws. She said there was no indication the worker was involved in terrorism.
Maloney said no disciplinary action had been taken against the employee, a mechanic he described as a SEPTA veteran, but the case is still under review.
"It appears to be a case of employee misconduct rather than a threat of terrorist activity," Jordan said. "I think he is about to begin taking vacation time immediately."
Also under investigation is how police responded to the discovery of the device.
The SEPTA police officer who confiscated the device on May 5 inexplicably stowed it in his locker for a week, smearing any possible fingerprints, before turning it over to a SEPTA police special-operations unit on May 12, Jordan told the Philadelphia Inquirer.
That officer may face disciplinary charges himself.