Is it a plain yellow flag? Or does it say something about lawn treatment/products on it?
Are you sure it's not a utility marker:
http://va811.com/homeowners/what-do-the-markings-mean/
If it is lawn treatment, we had this happen to us. We were able to find out the company and complain. They were spraying for an HOA nearby us and went a little too far. Very frustrating because we also do not treat our lawn.