Bird
I just thought of another one from the same time period. This was at home, not at work, but I had just moved to a new home in a new town and every night in the darkness, I'd hear a soft four-note tune being whistled near the property line. It was constant, whistling every fifteen minutes or so, every single night for a couple hours.
This terrified me for months (No, I never went to check it out) until I moved out, and the neighbor told me she had a parrot who would whistle from her shoulder on their walks around the property every night. Fuck off with your bird, Jen. I almost called the police multiple times.