It’s hard to explain without video but basically what you need to do is corner subdue someone on Rocco’s balcony while crouched in cover behind one of the corners. Either Renzo Forte on Landslide or the sleeping guy on The Author work for this (you could also do it with Rocco on World of Tomorrow, but since the mansion is open that’s kind of pointless).
Lure the person toward the corner where you’re hiding, and just before they get in corner subdue range you need to tap the opposite direction (so that 47 faces away from the corner) and corner subdue the person around halfway through the turn animation. This will usually glitch your location in some fashion, not always where you want to be though. When I did it on The Author I only got as far as the top of the railing on the balcony, but that was enough to drop onto the mansion grounds.
To get back in bounds, you need to get onto the observatory roof and find where the outdoor chimney should be. It won’t be there, and you can walk off the edge of the roof where it should be. Your goal is to land on the railing of the observatory and walk all the way around it, make a drop onto the railing along the cliffside, and then drop down the cliffside into the area with the Sanguine shop.
You can also get out of bounds on The Icon but you have to do it in a different way and there’s no known way back in bounds after that. You can access both the mansion and church areas on The Icon, but clutter blocks all the paths back onto the main part of the map and as yet I don’t think anyone’s found a reliable way to get on top of it and drop down.
Using the same technique on the edge of the Bangkok dock area, near the doors to the basement, can get you out of bounds on either map and into the riverbed. However, the riverbed is very unstable and there’s a high chance you’ll drop through the world. If you can get under the docks and to the beach safely you can run around there for a bit, and there’s a way to get back in bounds on the garden balcony nearest the corner, though it’s tricky. Same thing roughly on Paris I believe, but there’s nothing particularly interesting outside. It is also possible, though extremely difficult, to get into the prisoner’s room on Colorado. Can’t get out afterward though, as far as I know.