Thankfully, when I returned to stare at the blank wall again, a security guard told me that it was indeed missing and I was not crazy. However it wasn't painted on the wall but was a giant backdrop on canvas (or some such) that had been attached to the wall. Recently the Santa Ana winds had ripped it up badly so it was removed around Christmas time. Only the painted blue strip across the top of the building remains and if you look closely at the photo above you can see a thin strip of material near the corner that looks to be held in place by grommets.
So for those of you who have seen the backlot mural, it's not there anymore and now you know why. For those of you who never saw it anyway, I'll show you something more exciting next time!