This is a touchy subject.... Is the resident using it for medical reasons and are you in a state that recognizes medical marijuana? If you are, has the resident asked for reasonable accommodations under ADA to allow it? I am concluding NOT...
If your property is any form of affordable, then the solution is simple: get the residents out on a noncurable breach of contract as they are breaking FEDERAL law (it is the federal government that subsidizes the affordable programs and establishes those parameters, so you must follow Federal guidelines, even if the state guidelines are more lenient). Although the state may recognize medical uses, the FEDERAL government does not and your resident risks losing their assistance, and you risk losing your tax credits. Photograph the box and document the context under which it was discovered (in the routine of normal day to day duties, your maintenance team entered to address a repair issue and asked you to come in and see for yourself). Then consult your attorney.
If you are a conventional property, or your property is in a state that recognizes the medical uses, if the resident makes a reasonable accomodation request, which should be denied as it is not generally considered reasonable for a resident to ask an owner / property manager to be a party to the breaking of a federal law. Get your attorney involved.
In either case, document the discovery and the context under which it was found, inform your supervisor and ask your attorney for advice.
I would be curious to know if further investigation or documentation by law enforcement (an independent third party) is needed in this case.