This evening my 2000 Expedition Eddie Bauer had something running under the hood after the key was turned off. It appears to be the device under the brake reservoir - stuck on. This apparently had been running for some time, since the battery was almost dead. The best guess that I can find online is a power brake booster. Any thoughts? I'll be charging the battery in the morning and taking it in for service. Thanks...

 

If you have rear air suspension then its possible that it could be the compressor that lifts that up and keeps it level. Check the compressor located on the battery side of the Expedition.