All utilities, including UGI, must mark lines prior to anticipated construction. Excavators or anyone digging must call Pennsylvania One Call at least three days before excavation. Call 811 is a good source of information regarding protecting utility lines.
Markings are required for any purpose including utility work, building a pool, installing a fence and so on. You can ask the locator (person marking lines) why lines are being marked in or around your area. Each color corresponds to what is buried below ground: