It sounds to me as if it was softmodded at some point but that it may have been removed and the MS dash restored. As Heimdall said what you're seeing is the 'game' saves those particular applications and the installer create. Just because they're there doesn't mean the Xbox is now modded.
Are there any SID or Splintercell/Mechassault/OO7 game saves with Linux logos in amongst the ones you have? That might indicate how the console was softmodded.
I'd also look at the bottom of the console to see if it has ever been opened.
If there is some hidden shortcut to the Emulators and it was done using the basic UnleashX shortcuts it won't be a combination controller button press. I don't know whether there is even a possibility of it being such an elaborate combination as you've been trying. You'll have take advice from others about that.
The only combination shortcut I know is the in-game re-set (IGR). Hold down L + R triggers and press Start+Back together. If it works that will simply reboot the main dash. But it may at least confirm the thing is softmodded because you should get an UnleashX logo appearing during the reboot.
Should there be any shortcut set up it will most likely be one of the following seven individual buttons: Y, X, A, B, Start, Black, White. Simply boot or reboot the Xbox whilst holding down each button. If there is a shortcut just after the Microsoft logo X (box) screen it should load. If the first button doesn't work try another and so on.