Yea, they probably hooked up a gauge to the schrader valve under the hood up top on the passenger side. If that is good then you can say everything from that point to the tank is probably ok. Then a visual on the lines which are very easy to see to the HPFP.. then after the HPFP you can trace to the start of the rails... to inspect it further would require you to remove the intake manifold... don't think they did that... even if you did, not a WHOLE lot you can tell visually from that point...
I am not sure where the sensor is that picks up the rail pressure...
If it was a leaky injector he would be having other problems too... stuttering while idle among other things...
Honestly Brad has mentioned with mine anyway, that there is logic in the ECU that SCT can't see that maybe at play with fueling as well...
Hope LMS can get to the bottom of it for you.. again... I have a spare LPFP, FPDM, and HPFP... if they come back saying you need one of them, let me know...