Devil 49's experience tracks with mine.
Key to the use of Emergency Vehicles with flashing lights to mark power lines works only if you know that is what is being done for you and you know which vehicles are the markers.
That brings up the real key to all of this....effective communication between the ground units and the aircrew.
Usually, the Ground Crew have the best vantage point for observing Wire Hazards as their workload while checking for them is the lowest.....and they are physically the closest to them.
Most places I flew EMS, the use of Emergency Vehicle lights were controlled to facilitate marking the LZ (the only white lights showing) and Emergency RoofTop lights flashing only on the vehicles marking the LZ and Hazards, all other vehicles had white lights off, Emergency roof top lights off, and only Amber lights activated on the vehicle. That makes it very easy for the aircrew to pick out the LZ and any hazards that are being marked by ground vehicles.
The one real danger that remains.....is the unseen Wires and the aircrew must remain on guard during the approach and takeoff for wires that might have been missed by the Ground Units.
Most Ground units carried Bean Bag Lights or some other way of marking the LZ even if only some one with a Torch, Wand, or other lighting device.
The real key is training and coordination well ahead of actual emergency flights so everyone is working to a common standard and same procedures.