Utility workers came in and spray painted our back patio, no one is taking responsibility. What can I do?

The house behind us had a gas line replaced or something. We have a brick patio along the side of our house and a long the back patio. Last weekend we noticed that someone had come in and spray painted, with white permanent actual spray paint, about 15 big white circles on the brick along the side and back patio presumably marking a gas line. At the end, on the fence there was a single yellow flag with USIC, 811 and Center point on it.

Now I understand that utilities can access yards for emergent or urgent reasons, and may need to mark lines like this for work. However, no work was done or needed to be done on our yard, and from my understanding chalk or temporary point is generally used.

This week I spent several hours on the phone bouncing between Centerpoint, 811, USIC and even the contractor that did the work behind our house, DigCo and no one is taking any responsibility or even acknowledging there is a problem. USIC came out and looked at it, and simply said we didn't do it and we would never use that kind of paint. 811 is mostly just "oh sorry that happened" and Centerpoint just said deal with 811. I highly suspect it was DigCo but they're just blowing me off and the guy was refusing to answer because he was off the clock (on a weekday)

I feel insane trying to be a detective, but can't get anywhere here. Any other ideas what to do for someone to come clean this up? I've tried paint thinners and scrubbing but this s**t is not coming off.