When you move a card from one area to another (except when drawing a card from your draw deck), you must reveal that card to all players to verify that it is of the correct type. Exception: If an effect says you are to move "a card" with no other description, you don't have to reveal it.
Dwarf of Erebor's special ability announces in chat which card was placed under the deck.
https://wiki.lotrtcgpc.net/wiki/Comprehensive_Rules_4.1#effect