Some female relatives (I think I understood) were buried on a somewhat lower level (first you go outside, down some stairs, then into a small room).

Mr. Ismail Mert (see comment below) is right about the cenotaph sarcophagi.
Regarding the title of ‘sultan’ granted to Osman Gazi Bey, historical accuracy commands to say that both Osman and his son Orhan were Bey (and their territory a ‘Beylik’/principality). The first Ottoman ruler to actually claim the title of Sultan was Murad I (1359-1389), Orhan Gazi’s son and Osman’s grandson.

Correspondent: J.M.Criel, Antwerpen.
Source: (among others) Wikipedia.