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. 

If they paid residuals it would be different,  they play those episodes endlessly..... 

While I think everyone has their own idea of what wealth is, personally I would feel much wealthier knowing I have a residual income stream coming in month after month.

Yes and now many banks here (the one I work at included) won't lend on commercial MF over 65% LTV out of residual fear from the last bubble. 

I am going to have to pay for remediation in a month when he moves out and I'm concerned he will continue to deny in-unit smoking and fight me on security deposit deductions.I found a kit online that tests for nicotine residue.  

Smoke residue sticks to the walls.

I was thinking of taking $40k and purchasing a duplex for $18k (generate $800/mon) and a single family residence for $15k (generate $500/mon) and use the residual cash for emergencies or repairs.

The rentals are going to build your portfolio for residual income and the flips will build your funds to continuously turn for profit and investments. 

I have been working in analytical fields for all of my adult live (pension actuarial student and IT/management consultant), and am looking at other ways of providing residual income for myself and my family.Thank you for taking the time to read my introduction.

Whatever residual nicotine remained on the walls and ceiling from tobacco use from previous tenants has now been covered up by your painting.Gail