pilots trained and qualified in the EC-135 will know that late in the flight the fuel in the supply tanks is what he needs to be aware of, since the way fuel gets into the engines (main to supply to engine)
That holds true for every minute of the flight....as fuel in the main tank really is of only administrative value.....lack of fuel in the Supply Tank is "Life or Death" value.
If for any reason....at any time after starting the Engine...or Engines...that the Supply Tank is not showing a proper indication for contents for that particular moment of flight...that is a sign of a very significant problem.
Add the Yellow and Red Lights that appear at various times...and you have secondary indications of problems if they occur other than when supposed to appear.