How to extract the difference map from the past?

edit

Hello,

I want to know if there is the way to extract the difference from the past.

Now I'm using move_base and gmapping.
I think that global_costmap in move_base is calculating the costs from "map"( generated by gmapping etc) and "sensor topic" ( like scan ).

By combining "map" and "sensor topic", my robot avoided people and obstacles which weren't there at the time of mapping, but are now on the path.

Next I want to extract the position where they appeard.
Any format is fine now. ( like OccupancyGrid and etc )
I want to know if there is the way.

Thanks in advance.