Hi Jacek,
I think it is being used in the project.
It might be still in use in a not used level type.... but that still exists within the project!
(or a level using this head has been hidden/ made invisible/ etc... select a level and try to change its type, just to see if the level head is not in use in a different level type)
Once you have made sure nothing is hidden or using the symbol, purge levels types (Manage > Purge Unused , then unfold Families > Annotation symbols > Level), and then get rid of the level head (via purge or straight into the browser)!
Cheers,
François
Francois-Gabriel Perraudin
BIM management and coaching