2009 4.0 V6 - empty coolant reservoir

Excuse my lack of mechanical knowledge (doing my best to learn). I bought this vehicle in May, and added coolant as the reservoir was empty. Had it into a shop I trust for a pile of work and they gave it back to me after doing a good amount of work, saying it more or less has a clean bill of health. This morning I turned on the heat and it blew cold air, so I checked the coolant reservoir after it sat most of the day and it was empty.

Filled it appropriately, but want to make sure I’m not missing something big. Radiator cap isn’t cracked, so could be a leak elsewhere but I’ve never noticed a leak. Is 4.5 months a worrying period of time for my reservoir to be empty? What else should I check? I do daily drive this truck.

It also didn’t fix my cold air issue. Suggestions for that would be much appreciated as well. Car has never run hot.