go to first floor of high rise flats must have locked doors !
then run double jump at the door this will let you jump through the texture wall doors. then walk round rooms that are out of reach or if you walk to one of the rooms on the left you'll drop below the base of the high rise flats. then you can see and walk around underneath it.
to get out you can either run and jump out of the back of the texture /walls or at the front where the steps join the main buildings. i can do this every time on all of the flats with locked doors.