For clarification, this is my mom's story from one of her part time jobs in the early 90s, so this happened a long time ago. I got her consent to post this as we read through this sub regularly.
In 1990-1991 my mom started working at a group home as a youth worker doing counseling and support work. For context, this home was in a residential area not too far from the local hospital/police station/downtown core. This home had 3 floors and had about 8 different girls who lived on the second floor, and had an attic above. The main floor was where the office and kitchen area was. At the time, the agency didn't have strict regulations, so my mom would be working night shifts by herself and having to manage all of these kids alone. One night my mom began her shift and was made aware of some really creepy information. Both the home supervisor and the kids who lived in the home reported weird smells at night, sounds from the attic, and food going missing on day and night shifts. Keeping in mind at the time, nobody was allowed up there or stayed there. My mom would have to do half hour checks to ensure all doors were locked, however during her shifts she didn't notice any problems. Thankfully for her, none of the weird events took place on her specific shifts. As events kept being reported, they then called in maintenance workers.
When the maintenance workers went around to check the property, they noticed that one of the windows on the third floor to a room which was locked had been broken. The room wasn't accessible from the inside of the house, so nobody knew. Unfortunately, that room had access to the attic. The maintenance workers then went up to check the attic and found various things. Food wrappers, clothing, and even various things from the residents bedrooms. The room that was locked off from the inside of the house was able to be opened from the other side, meaning that at some point throughout night or day shifts, the stowaway was walking inside the home, and no one knew. They would've had to have come down when the residents were either being taken to school, asleep, or at appointments, but they also were able to view the schedule board in the office. They would've known when staff were coming and going and any days when residents would've been gone.
During the time that the stowaway had occupied the attic, there were reports from other staff members about them feeling as though they were being watched. Eventually the maintenance workers fixed the windows, the fridge was locked, and security cameras were installed. There wasn't any issues after that, however my mom does have other creepy stories after working 20+ years in the field.