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The old marks aren't being cleared

behind new observations.

The object moves toward the sensor in the 2nd frame. Since the object now obscures the sensor from seeing behind it, there is no simple way for the costmap to be updated to clear those occupied cells. The way for the sensor to update cells as "not occupied" is to have a sensing beam sent through the cells and not reflected back. This is what the clearing: true parameter sets in the costmap configuration files.

In the 2nd frame, the object moves closer so the sensing beams are reflected back but haven't had a chance to fly through the previously occupied cells. This may not be ideal for your use case, but in general its safer because if you cannot perceive what's behind an object, you can't make guarantees about whether or not that space is occupied.

If you want to track moving obstacles like that, it will require more work- recognizing it as a dynamic obstacle and updating the costmap cells directly.