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I've had that happen to me before as well. I had the NAV screen up and realized that I was listening to FM instead of my Sirius (I swapped out the XM unit with a GM-SIR1, took about 15 minutes). When I hit the source button, no XM buttons! Pulled over, turned the car off, restarted, had my XM buttons back. Very wierd and only happend once so far. I was worried it might be an issue with the Sirius, but since someone has reported this with an XM unit, I'm thinking this must be a bug in the head unit.
I saw that when I swapped to sirius once and after a restart it was there. Guess if removed it does not sense it and takes the button away just as if you added a mod for an aux input and now you will see aux. XM and Sirius are merging anyway so might as well stay with XM and they are cheaper at $77 a year after haggling.