Sorry I have felt bad for a while now about this. I work for the Microwave industry and follow travel trailers around on a regular basis. See when you are on a long stretch of road going straight I shoot a rope over to your rig and slide across to get inside and either break the microwave out right or I go for the replaceable at a not so reasonable price glass.
Sorry its what they pay me to do. I imagine many of you right now imagine the music in the back ground to be mission impossible but Ive done it 100s of times its not that tough.
My vote is that someone pulled it out to wash it and never put it back in as was already mentioned.