Tried it again this morning waiting longer this time and the X5 started up. As far as what may have drained it, we're pretty sure it was because the interior lights got 'turned on' by the window installers when they installed the new window and we didn't notice it until that night. Pulled the spare out to have a look at the battery - it was a BMW battery but I don't know if it's the original since the only label just had BMW part #. Also noticed the 'indicator' light was black so wondering if we might be able to just get it re-charged or it's time to replace it.