Approaching hidden items will cause a response above the heroine's head. The closer one gets, the stronger the response. An action command will appear when one is at the location of a lost item, and performing the action will allow them to obtain the item. If Aria's action level is too low, obtaining it will not be possible.
Any lost item, once found, will be recorded on the Mini-Map, so it is possible to try again when her Action Level is higher. Also, Action Level rises when particular quests are cleared.