I have no reason to believe there is any moisture issue from the building itself (and I've had more than one contractor come out, two even that put moisture readers on the walls and ceilings with no abnormal results).My question is, what steps should I take as a landlord?

Any framing against cement walls must be pressure treated, contain moisture barrier, and insulation.

I have seen that many,many times and it is in many cases a cash flow killer.If tenants do not pay or reimburse even they do not report water leaks which can cause higher water bills, mold, water damage to walls and floors, bug infestation from the moisture growth,etc.Tenants also use about 30% plus more resources on average when they are not paying directly.If tenants paid directly and water drips once every 30 minutes from faucet you better believe most will let you know right away!

The water has not been turned on in 1 year so I am unable to see where the leak/moisture could be coming from.  

Run a knife over that to knock high spots and a gentle light sand with the pole and screen paper

They also add a lot of moisture to the air and they recommend a window be cracked open.

Please be gentle...Remember, I'm new : )

We have gently brushed on the idea of selling but are still building cash flow 6 months into the deal.  

Moisture getting in through window.

Ensuring consistent soil moisture content is key in this area.