I know basement is prone to leak so I did some homework.  

Every gas leak I ever had to deal with was a loose fitting. 

I’ve also had 3 main line leaks between house and road.

She feels that leak is causing the downstairs bathroom sink to leak as well.

If it matters, I pay heating (gas) for the entire home (one meter) but tenants pay their own electric.

(Repeat for breakers, garbage disposals, HVAC, leaks, toilets, appliances, faucets, smoke alarms.)A Universal Catch-All Prompt“ChatGPT, act as a professional home-maintenance technician.

One thing to consider, is it possible the P traps are drying out or possibly leaking?

The sewer pipe was mostly not leaking even though it was completely broken on top.  

We had someone come out and as they were clearing it, a leak developed into the lower unit. 

Hey Ruslan, it's obviously all depending on the age and consistency of maintenance of the property and equipment, but most likely plumbing issues with leaks and clogs needing to be snaked, roof leaks especially around certain openings like vents and skylights, HVAC issues like filters needing to be changed and duct cleaning, and even silly things like smoke alarm battery changes that some tenants likely call about.