My advice in another comment was to take a shop-vac and run it as a blower, then stick the hose onto the conduit/h-box. Have someone else with the panel cover off listening/feeling for air coming out of any conduit.

This is a concerning one for me though. Removing/replacing the cover could be problematic. It’s residential, and I see spare data conduits being left by contractors as… a dubious event. At least outside the context of custom homes.