Search for the elements, which are not used in diagrams, finds attributes, operations, or slots that are represented on transitions. How to find out whether a particular element is needed in the model or not?
It is considered that an element is used in a diagram, when there is a dedicated symbol representing it on the diagram. States and transitions have dedicated symbols for them. When a signal is assigned to the transition via trigger, it does not have a dedicated symbol on the diagram. It is only a part of textual representation of transition signature.
Let's do the following: assign a type X to an attribute on a diagram. Though we see X on the diagram as an attribute type, it is not considered as used in the diagram, because it is displayed only as a part of the attribute notation.
If you want to find out whether a particular element is needed in the model or not, use the Usages/ Dependencies functionality. Select an element in the Model Browser and from its shortcut menu select Used By. This way you may follow the chain of elements in the model and decide whether a given element is rubbish, or it makes sense.