I don't think you need to worry, it's just water vapor.

It drives me nuts when people come on looking for advice and then vaporize into the night.

Could see recent sewer line work - i could not find permit for, hole in roof, very little insulation in attic but plenty of racoon stuff due to a 2 foot hole in back of roof over bathroom,  they had put new carpet and paint in all rooms except the bathroom which all sheetrock was pulled down, surround pulled out,  mold on vapor barrier and adjacent areas, floor rotted and a wonderful smell.  

Vapor/smokeshops seem to be big recently.

We also own a couple of vapor stores-- our son bought my husband an ecig for father's day four years ago and he managed to quit smoking!

Yet for many people who don't have access to these things, marijuana with both THC and CBD components are the only thing they have access to. 

If so, and if there is only one unit testing positive, that is likely your common area smoking unit.Check this out: https://www.amazon.com/THC-Surface-Residue-Detection-Test/dp/B00BPX4XW2I've never used it and don't know anything about it. 

My "velvet glove" approach would be to light-heartedly tell them it's getting too loud outside and it is time to buy a vaporizer.

That and having your kids exposed to THC and belligerent homeless is not a welcome mat.

This paper backing is a vapor barrier and will trap moisture in the attic and you will have mold from the moisture trapped by this.