Which of the following indicates that aircraft engines may be running?

The correct answer is that the beacon is flashing, which serves as an important safety indicator that aircraft engines may be running. The flashing beacon is a visual warning that alerts ground personnel and vehicle operators to be cautious and aware of the potential presence of an aircraft in the vicinity with its engines on. This signal is crucial for maintaining safety around active taxiways and runways.

In contrast, the other options do not serve as reliable indicators of engine activity. For instance, the removal of cones typically indicates that an area is clear or that certain operations have been concluded, rather than signaling engine operation. Similarly, if the lights are off, it does not provide any indication about the status of the aircraft engines; the lights can be off regardless of whether the engines are running or not. Lastly, the absence of a marshalling crew does not inherently signal whether engines are on, as there may be various operational circumstances where they are not required or present, independent of engine status. Therefore, the flashing beacon serves as the critical signal that points to the engines possibly being in operation.