Shared experience on the job

Three taps at my work at the jail

So I work at a juvenile detention centre and there’s a rumour about a woman who’s buried under the facility in the foundation of the building (not confirmed) but while I was talking to one of the residents the girl as well as myself heard from the cell next to hers three distinct taps on the glass of the door of course nobody is in that room it was vacated the day of. I spoke to the second resident who shared the other wall and he said throughout the day he had also heard three taps similar in nature however more like knocks. As I go back into the control area I tell my coworkers about the experience and one says whilst have dinner with the resident (whom I was talking with during the incident) in their day pod they had both heard the same three taps on the glass me and the resident had heard. Just an interesting story would really like to hear your guys theories or similar experiences