I've discovered 2 spots to go out of bounds off, one requires a bit of parkour setup on some railings, and the other just needs landing on a railing with a broom and jumping off. both spots are at the exterior of hogwarts. also you will need a broom for being able to actually go exploring under the map, since you will inevitably fall onto an invisible plane where you get stuck/softlocked in, if you dont. when youre flying under the map, the game thinks youre on the outside, so most rooms of hogwarts become unloaded, so you can freely roam inside the empty castle. from there on theres probably a bunch of possibilities to skip into locked rooms etc, since you can land on certain areas that are still floating around in the castle. once you get of your broom in the castle, the rooms will load back in and in most cases you will be forced inside. i was able to, for example, clip into the common rooms of hufflepuff and slytherin as ravenclaw.

clipping into ravenclaws common room is possible too, so gryffindor would be the pick for a true 100% run, since you could grab all the chests from the other houses that way