Why doesn't costmap clear the old obstacles outside the scanner scope?

edit

don't include obstacles layer in the costmap_common_params or global_costmap_params insert it only in local_costmap_params

I mean this part or alternative depend on how you configure your obstacles layer

observation_sources: scan scan: {sensor_frame: base_scan, data_type: LaserScan, topic: scan, marking: true, clearing: true}

however it'll effect global path planner behavior. otherwise you can clear using this service

rosservice call /move_base/clear_costmaps "{}"

If the sensor does not see new objects, it will not clear the object out of the map and retains the previous valid information it received. The costmap will retain the previous data (retain the presence of the previous obstacle) until it gets fresh data from it's sensor indicating that there is no obstacle.

One way to get around this is to create a node that would fill the rest of the laser scan with max range (~20m) so that the maps would be cleared. This is code for the node -

https://github.com/clubcapra/Ibex/blo...

However you must keep in mind that it isn't completely safe - if something blocks the laser or it fails your robot will drive straight into things, which is why it's not the default.