If someone travels to for example Turkey (could be anywhere) and stays for 90 days could they within that 90 days visit another country and then return to Turkey ? 

You obviously need to give details to the UK police that your travelling to Turkey and intend to stay for 90 days but do you need to give them details if you visit another country for a few days if you still return from Turkey on your expected date ?

Would you need to even tell them you visited another country or even 2 other countries when you returned ?