Marijuana smoking, in addition to cigarette smoking, has become VERY common. Many older people , 65-80 year olds are using it. They’re vaping, etc. I rented out my single-family home that I was living in because the 65-75 year old neighbors were smoking in the backyard, and the smoke would enter into my home through the ventilation built into my home (through the roof vents, the crawl space vents, closed windows, etc. They had physical/mental health problems and used MJ to manage it.
I have asthma, severe allergies and chemical intolerance, and I had several large BlueAir air-purifiers, with Dual Protection filters to filter the smoke, but I nonetheless still ended up in Urgent Care in2022, during the week of Labor Day holiday. Three times. The cops couldn’t stop them, so I decided to rent it out and am now looking for a place (and ADU) where the neighbors adjacent don’t smoke.
The message is: your ‘accuser’ tenant could have asthma / allergies and/or chemical intolerance (chemical sensitivities), which is the reason she/he is speaking out. The air-purifier in the unit may help, but not prevent their health problem. The older couple could indeed be smoking MJ inside their unit, away from the front door, so you can’t sniff it out. Also, if you, or others, don’t have chemical sensitivity, and if the older couple have tricks up their sleeve for removing the smoke quickly, you may never know that they’re smoking.
However, smoke does travel through vents, electric outlets, even through plumbing, and it enters into others’ adjacent units, and homes, and if you aren’t physically sensitive to it, you won’t get a reaction.
Your ‘accuser’ likely has severe chemical sensitivity, and they’re reacting to it. That person is in a very difficult position because they may not be believed.
I wouldn’t be surprised if the older folks above are smoking. However, if your apartment complex is located near another, adjacent apartment complex, or home, where someone is smoking, the smoke from the nearby, adjacent apartment complex tenant(s) or single-family tenant could easily enter into this person’s unit, through windows, vents, and any crevices. I wanted to share the latter, too.
Older age groups (65+) are now one of the biggest consumers of MJ. They’re retired, experience more physical (and/or mental) ailments, and they generally have more cash and time to spare. Regardless if others pick up on the smoke or not, the MJ and cigarette chemicals do linger on surfaces, walls, furniture, and even on sidewalks, where their marijuana smoke may escape through their windows. The chemicals also stay on the smokers’ hair, and clothes, and in their car.
If you want to investigate further, you ca. with the above info.
Your ‘accuser’ may not be insane. She/he may be experiencing severe reaction to the MJ smoking and vaping nearby, from either the upstairs couple, or, potentially, from smokers a next-door, adjacent apartment building or home, where there’s a smoker.
Smokers are addicts, and they’ll smoke as frequently as 3-6 times each day, per smoker, and if you’re a physically vulnerable person, with documented asthma, multiple chemical sensitivity syndrome (chemical intolerance) and allergies, that chronic smoke will send you to Urgent Care or the ER, even if others nearby don’t ‘smell’ or react to it. Everyone is exposed to the MJ smoke/vape chemicals, but only the ones with a moderate-severe diagnosed sensitivities will get a big enough reaction to voice their concern.
P / back off, I believe. Please stop 
letting these renters run you around; you have More Than done your due-diligence.