That would hold true for non-regular users, however, a regular or semi regular user would show trace full term.
A heavy user will show high trace levels.
Police/Law Enforcement tests for intoxication test for trace.
Now, the Medical Examiner, working on the Room Temperature user, the tests get a bit more detailed.
I believe Gas Chromatography will show the level of psycho-active Delta-9-THC in blood samples. Delta-9 will fall beneath detectable levels within a few hours of marijuana use.
So we know she was using within, say, 3 hrs. of the crash. The actual level of Delta-9 THC in her blood will tell investigators how much withing that time frame.