My guess is that you are not getting the signal from the satellite, you are picking up the signal from a terrestrial (sp?) repeater. Check the signal meter in your unit, and it will tell you whether you are picking up the satellites or a repeater. It is not unusual to get a signal in some tunnels and garages. The weirdest place I got a signal was in a tunnel underwater. I also had cell phone coverage in the same tunnel.
Depends on where you live. If there are terrestrial repeaters by you, then you're okay. But if not you'll have to position the antenna so it can "see" the south sky.
They do sell a repeater kit if your XM radio will be in a spot where you can't get a signal.