My 2005 X3 w/120k miles; running fine; service engine soon warning light came on

you will need to have a scanner to turn the light off as the newer vehicles have a computer with a keep alive memory. this means that when the battery is disconnected it will not clear codes on the DME control module. Most shops will perform the scan test for a nominal fee

Hard to say without any data such as fault code it could be anything... Hope you follow up with some more data or visit your local service station then post up their findings. Best of wishes Michael