Her back yard is packed with all the stolen plants and other things from the neighborhood, so there is physical evidence. I have had stuff stolen from her myself and gone to the police, but I guess its not important enough for them. Her family just acts like they don't know what's going on.

There are gas fireplaces that require ventilation and gas fireplaces that do not (ventless). You will need to check whether your particular unit requires ventilation. Try googling any info you can find on the unit, and you should be able to find the answer. It may also say on the unit.

My guess is that it is a vent running from your hot water heater or furnace in the basement up to the roof. The emissions from these are often vented up the old fireplaces to the roof. I see that someone recommended Safwat. I have used him and recommend him also.