I agree, battery seems the most likely.

You may want try replacing the battery in your key fob (as you have comfort access). You didn't mention a second key fob?

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2008 X5 4.8i | Wheels: Summer> style 336 20" RFT; Winter> style 214 Ferric Grey 20" RFT | Aero kit | ESS E-Flash tune | V1 | LCI Rear Tail Lights | Coding completed | Engine replaced at 90K