R56 2013 Coolant issues

[speedo]

Got an oil change done Tuesday his week and the shop told me I had a minor coolant leak. Drove home in 100 F heat with the AC on. Today when backing out of the garage, I noticed a sizeable puddle of coolant and when checking the reservoir...I noticed it was low, so I topped it off. I started it up and could not find a source of the leak. There was some visible antifreeze on top of the trannie. I put the mini on my lift, took a pressure tester and at 12 psi could not find any source of the leakage...either above or underneath. Went for a short drive and put it back up on the lift, noticing minor antifreeze around the bottom of the trannie...on the left side of the engine. With a light and inspection mirror, the only area of seepage was where the drivers side heater hose (that goes through the firewall) connects to the thermostat. Below that connection is the general area that is wet. I am wondering several things... why no leakage under pressure? Why was the leakage amount considerably higher when it was hot and I was using the AC? Is the leakage at the thermostat connection typical of a failed gasket or O ring or bad thermostat, or just a leaky connection? Why is that heater hose connected to the thermostat at all? Is it more likely that the hose failed or the thermostat? How difficult is it to swap out the thermostat?

[speedo]

So I pulled P0597 code from the car. Thermostat open code. Sounds like an issue with the thermostat...but that doesn't explain the leak. Maybe the thermostat is intermittent? Could it have been stuck closed and then with the AC on and 100 degree temps...it suddenly opened? Again...no leaks when pressure tested, smallish leak while running. Thoughts? Could the technician have bumped something, or disconnected something when replacing the oil filter? This is a strange coincidence...no leak before oil change, but now I have a leak?