You need to get to the source, I'm afraid. We started getting the more than occasional housefly buzzing around, but after ineffectually chasing them down individually, I didn't solve the problem until I discovered that the ones I saw were just outliers from an appallingly large colony descended from someone who'd gotten in through the patio door and were now living along the baseboards there. I stomped those all out and that solved the problem.

I presume from the description that your piano is an upright. We once acquired an old school piano and fortunately had no problems; when we inherited B's mother's piano and passed along the old one to a FoaF, she didn't ask whether it was infected either, though from your story it sounds like a good thing to check for, even if the answer is, as it was in your case, an incorrect "no".