Slowly take a tour in the environment and make closed cycles, you will notice that the grid map is being drawn along with some loop closure constrains is been detected.
After taking a tour you will notice that your map is saved to ~/.ros/rtabmap.db
You can examine this map database using rtabmap-databaseViewer ~/.ros/rtabmap.db
Here is the rtabmap.db database so you can examine it.
Screenshots
figure 1: Updated robot and environment
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figure 2: RTabmap Database viewer showing samples from the resulted database.