Wow, I was gonna ask the same question!
I have manual HVAC as well, '09.
I was trying to save gas on the way home yesterday so I could fill up this morning. I turned off the AC after I got it just comfortable enough to drive with the windows up and set it to recirculate. The air stayed cool so I wondered about it when it didn't start to warm as I drove down the freeway. At my exit there is a light that stays red a long time, so I shut the engine off and turned the key back on. That's when I noticed the air warming and getting more humid like normal. When I restarted the engine it switched to outside air without AC, but when I set it to recirculate the AC came back on, and like the OP said the AC light was off.
Is this something that the BCM programming does when it detects elevated temperatures, or just a hinky thing that GM decided on? I'd like to know when I turn off my AC that the AC is off when I do anything else, except for the top 4 clicks of Defog where it automatically runs the AC.