Does a genotype test detect resistance mutations only to drugs I am taking at the time of the test? And could resistance to a drug I took years earlier then be present but undetected?

Yes to both questions. HIV clincians know to instruct their patients to remain on their current regimens while they obtain a resistance test, so that they can see the effects of the specific selection pressure exerted by their current regimen.

As you suggest, viruses with resistance mutations commonly 'revert' to wild type virus in the absence of that selection pressure, so a resistance test is less useful in patients who are no longer taking their ART.

Which is not to say that tests on patients off drugs are totally useful. In some cases, resistance mutations persist for months or even years following the cessation of treatment. And we routinely recommend drug resistance testing for patients who are recently found to be HIV positive to see if they have acquired a virus with one or more resistance mutations.

'Archived resistance' refers to the presence - at low levels below detection - of one or more clones of resistant virus that developed while on one drug (for example, efavirenz), but which then reverted to wild type when the EFV was discontinued. A resistance test performed after these events would suggest that the patient is susceptible to EFV...but if that drug were resumed, the clone(s) with the EFV mutation would quickly re-grow to become the dominant population, and the regimen would fail. For this reason, clinicians have to know the full treatment history and the response to treatment when interpreting a resistance test.

Talk to your doctor about these issues.