Haha that beeping in a Air freshener unit running out of battery the tunnel is outside in the field, by the goal post..there’s a gate that leads into the tunnelsNice, sounds like maybe you got lucky.
When I went in late February it was the same. I spent hours there but still had loads to see, so wouldn't mind heading back. Apparently there's a tunnel somewhere but I totally missed that...
I did hear a bleeping noise in the main lobby area when I was there thinking about it, but it seemed to be coming from an electric air freshener in one of the bathrooms!