Which of the following is an abnormal pipeline facility condition?

Missing signs or markers represent an abnormal pipeline facility condition because they compromise safety and regulatory compliance. Signs and markers are crucial for identifying the location of pipelines, informing about potential hazards, and guiding in emergency situations. Their absence can lead to accidental damage by excavation activities or other unintentional interference, putting both public safety and the integrity of the pipeline at risk.

In contrast, aboveground pipes on supports and properly functioning valves are typically part of standard operational conditions, reflecting acceptable practices in pipeline management. Authorized encroachments also indicate that necessary permissions and safety considerations have been addressed, further underscoring the importance of operational integrity. Therefore, the absence of signs or markers stands out as a clear deviation from expected standards for pipeline facilities.