National Museum Cardiff

The ghost of the architext responsible for this building, Arnold Dunbar Smith, is said to manifest as poltergeist-like activity with objects and furniture being moved. His ashes were placed on display in the museum but were moved to make way for new toilets. This is said to have angered him!

Update 1/9/21: I received the following email from the museum via Facebook: "The museum building is not especially old, approximately 100 years, and does not experience any "inexplicable phenomena" on a regular basis. I'm not aware of recent "odd" events."

Link: Wales Online
Link: Higgy Pop