Anyone using the Minut noise/cigarette smoke monitor? 

Smells of cooking or smoking are likely to do so as well.Having said all of the above, the flats I'm familiar with have rented reasonably well at market rents.

They dislike the new tenants in front and have complained about smoking (not allowed) and loud fights.

To add the unit you will most likely need to achieve proper fire rating between shared walls/floors/ceilings and have interconnected smoke detectors.

He had smoke detectors in all rental houses.

Requiring renter's insurance (separate from your property insurance) is also very important since they typically would not have the cash reserves to pay for a major damage such as water damage, ruined floors, fire/smoke damage, and you will be sued by 3rd parties if something happens to them while at the house if they have to go to the ER. 

The device also detects smoke and has saved a couple hosts from a smoke filled and smelly STR. 

The window must have a net clear openable area of 5.7 square feet (5.0 square feet if ground floor), a minimum openable height of 24 inches, a minimum openable width of 20 inches, and the bottom of the opening must be within 44 inches of the floor.Size: The room must be at least 70 square feet, and no less than 7 feet in any horizontal dimension.Lighting & Ventilation: A natural light source (window) of not less than 8% of the floor area and natural ventilation (openable window/door) of at least 4% of the floor area are required.Heating: The bedroom must have heating capable of maintaining a minimum room temperature of 68°F.Smoke Detector: A smoke detector is required in each sleeping room.While a closet is expected in most bedrooms, the International Residential Code does not actually require a bedroom to have a closet.

If the tenants are staying usually the only initial repairs/renovations we do while they’re still in the unit is to upgrade the smokes/CO detectors to hard wired and anything else that turned up as a life-safety issue in the inspection.

Well, it worked, but only because it was already a smoking deal and I was just trying to shake a few more pennies from the cushion - otherwise, I would have walked and had my agent tell them to sell it to the other offer ;)On the plus side, I now know how this particular agency operates, and I will avoid their listings in the future.