ATS found!
Thanks to all who helped locate this unit! My wife, after trying to dismantle the bed, said there is no way they could get to a unit UNDER the bed, so we opened the outside compartment door (at the end of the bed). There, hard to spot, was a full, thin panel wallpapered like the wall it was attached to with 6 tiny screws. With the panel off, there was the ATS right in the middle and surrounded by a number of small junction boxes.
Thanks again, everyone.
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