Leadership transfers if the clan leader doesn't login after a certain amount of time.  I think it's 30 days but Id need to double check.

Thanks for answering. This raise for me the question if an active clan leader who wants to stop, can transfer the leadership directly to one of his clan members?

Yup.  Clan Leader can directly transfer the role if they wish.

Subtle, bot, very subliminal. 😋

I always check my vault if I can't find him. 

Super sneaky 

Thanks all for answering.