previous scanned map areas disappeared

edit

Are you using laser scans? You can also check the 3D map projection /rtabmap/proj_map from the Kinect, which fills empty spaces by detecting the floor.

For the map created from laser scans (/rtabmap/grid_map), by default only empty cells between the robot and the obstacles are set as "Empty". If the laser doesn't hit anything, the cells remain "Unknown". On version 0.10.5, the parameter grid_unknown_space_filled can be used:

<node name="rtabmap" pkg="rtabmap_ros" type="rtabmap">
    [...]
    <param name="grid_unknown_space_filled" type="bool"   value="true"/>
</node>

The unknown space between the obstacles should then be filled by "Empty" cells, up to laser scan range.

There is also the parameter map_filter_radius that can make some parts of the map disappearing, you can set to 0 to disable the node filtering. See rtabmap_ros doc.