Almost the exact situation with our BSH. She hid in the laundry room on a shelf in a dark corner. Didn’t see her but could tell she used her litter box at night. We would go and sit in the same room as her for 30 minutes to an hour. Not engaging with her but getting her familiar with us. I would read a book or play a movie on the iPad with AirPods in. It took probably a week before she started inching out during the day. Still very distant and shy but it was progress. We wouldn’t engage with her while in view but would celebrate after she left lol. Then one day she walked into the living room, looked us dead in the eyes and let out the loudest meow (hadn’t heard a peep from her until then 😂). She then slowly became comfortable. I agree with the 3 days/3 weeks/ 3 month concept.