What caught my attention is the fact you said, you logged in and never logged out, that is to say, there is every tendency that someone with the knowledge of cryptocurrencies have accessed your wallet and not the exchange. You might have given out your phone to a friend and forgot you never logged out and the person took advantage of it. I will suggest you check the date the transaction was made and try to think who you met on that same day, if possible recall all activities. As for the funds, nothing can be done about it.