For curiosity, if an app is running (in my case it is indicator-multiboard), I deleted it using sudo apt purge indicator-multiboard successfully, however it is still running over the task bar, so I tried sudo apt autoremove to see if there is any dependencies, but not.
So why is this happening?
Programs get loaded into RAM and operate from there (because RAM is faster than hard drives and other storage types). You have uninstalled the app from disk when you did sudo apt purge indicator-multiboard, but have not stopped the copy actually loaded in RAM. Once you exit the application, it will be gone from RAM ( although IIRC some cache may remain in swap, which will be gone once those parts of RAM need to be freed up by the system )
If the application indicator-multiboard doesn't have an exit or quit button, you may consider killing the process via pkill -f indicator-multiboard or to be safer - find out its actual PID via pgrep -f indicator-multiboard | xargs ps -p and then use the PID you find in kill command. Of course a simple reboot will have similar effect (on server it's not always possible to reboot, but on desktop it's more than acceptable).
Side note: deleting app from hard drive ( or any file for that matter ) is like removing book from catalog in the library - actual book is still on the shelf until librarian ( the system ) decides to put something else on that shelf and mark it in the catalog.
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