The neverending beep

While working on the deck, I usually have my ipod and headphones on. At one point last weekend when my ipod was not on, I happened to notice an audible beep coming from somewhere near the deck.

At first, I thought it was a fire alarm somewhere inside my building. I came inside, opened the door to the back staircase, and shut the windows. No beep. Returning to the deck, I heard the beep once again. I concluded that it was probably a fire detector somewhere amidst the dense collective of buildings visible from the deck.

It's now ten days later, and the beep persists. It probably beeps every minute. How can the owner of the building with the beep not notice this? Or, at least the upstairs or downstairs neighbors in said building.