Advice - bats keep coming into house

Hi there

Looking for some advice. For the last 3 nights we have found a bat flying around our bedroom (we live in a 1st floor flat in the middle of the Ladder, our bedroom is in the loft conversion). We saw the bat fly out through the window each time and checked the house thoroughly so its very unlikely to still be there during the day. 

After the second time it happened, we ensured to close all the windows in our bedroom, but the bat managed to get in again (we found one tiny window open downstairs that we forgot about).

I understand its common for young bats to accidentally enter houses in the summertime but as this is a recurring event, I was wondering if there is any advice on how to prevent this and possibly find if there is a bat roost somewhere in the building? We have three fireplaces (which we dont use) which are sealed so are at a loss as to where and why the bat keeps entering.

Thanks!