A worker noticed the dirt-caked turtle moseying along.
Assistant curator Leishawn Spotted Bear says Bob apparently had been with a group of turtles to be moved last September as the staff vacated the museum building.
But she believes the 15-pound turtle went underground, at least for awhile, when his fellow reptiles were relocated to another part of the museum complex.
The museum building was demolished and construction crews have been working on a new facility.
Spotted Bear thinks recent heavy rain loosened the dirt where Bob had been.
She says Bob appears a little thin, but is doing well and will get some treats, like minnows, to "fatten him up a little bit."