You said it, hard to pinpoint. Maybe the previous shop overfilled the oil a bit so the warning never came on. Maybe the new shop went a little or a lot shy of full and that's why it happened. Maybe you have an oil leak. Typical for this engine to have valve cover gasket and oil filter housing gasket at some point. It is more than likely nothing to be overly concerned about because one quart at 3k miles is not the end of the world. No one here is going to be able to answer, just guess like I am doing right now. We're reading tea leaves.
Next oil change ask the shop for the exact oil they used, exact amount they put in, and if they see any obvious oil leaks in the common areas for these cars.
Also, motor oil is not like a gas tank or windshield washer fluid. At full capacity there is only a portion of the entire engine block that is filled. Adding a quart is not going to make it overflow.