Well, that will be critical in differentiating certain medicines.
All decongestants are chemically similar to amphetamines, so any medicines that contain decongestants can cause a false positive urine drug screen for amphetamines or methamphetamine. That would include certain cough syrups, cold and allergy medicines, and certain nose sprays. However, if the doctor orders a confirmatory test for a positive drug screen, the confirmatory tests will readily identify that there are no amphetamine or methamphetamine in the system.
You are also correct that urine that is less dilute because of dehydration will cause higher drug concentrations of any drug, including decongestants, and would be more likely to cause a false positive urine drug screen, but the confirmatory test will still clarify that no amphetamines or methamphetamine are present.
The Adderall is a different issue. Since Adderall actually contains amphetamines it will not only cause a positive urine drug screen, it will also cause a positive confirmatory test.
Most doctors will typically order a confirmatory test whenever there is a positive urine drug screen, but if you are contacted by the doctor or his office about a positive result, the first question that you should ask is whether a confirmatory test has been done, and if not you should request that one be done because you know that you are taking medicines that can cause false positive results on a urine drug screen.
If a confirmatory drug test is done and is still positive, then that can occur from accidentally taking Adderall, and you would need to explain your situation to your doctor. But the odds are far more likely that this is a false positive from a decongestant, and getting a confirmatory test done will clarify that no amphetamines or methamphetamine are present, and there will be no further issue.
If I can provide any additional information, please let me know.