UPDATED: 5:56 p.m.
Numerous buildings were evacuated as a precaution shortly before noon, from The Bellevue and Rittenhouse Square to 1500 Market St.
A police officer told KYW Newsradio that the odor came from workers cleaning out tanks at the former site of the Philadelphia Energy Solutions refinery. That odor made its way to Center City on this windy day, and ended up in buildings' ventilation systems.
The Office of Emergency Management said in a tweet that the cause of the smell was "a release of mercaptan, a non-hazardous substance, during equipment cleaning." Since natural gas is odorless, mercaptan if often added to make the gas easier to detect.
Thiel says as Center City buildings, including the Municipal Services Building, were evacuated because of the odor, PGW got 500 calls from Lawncrest to Southwest Philadelphia about the smell.
He says the PES sensors did pick up some product in the air that seemed to cross the refinery property in the morning. But Thiel says a check with New Jersey DEP and other area agencies failed to solve the mystery.