Some things come to mind, and forgive me if I am presuming these were thoughts that you haven't had.

Get neighborhood KIDS involved. Us adults are usually so busy doing too many things to really focus. The kids love to help, and can be the eyes and ears many times over.

Re-poster, re-call, revisit the clinics/shelters and any place you might have put up a flyer. People 'see' but don't really see the same sign day after day. Put up something new and fresh- throw out the reward and 'needs meds' words.

Remember that Butter might be 'hiding'..and dusk and dawn are key times for spottings.

Finally, if there are other homes in your community that have indoor/outdoor cats...go to their houses directly. First, you have their sympathies as a cat owner and more importantly...they would know if a strange cat has come to their door/window.

Good luck. Butter, PLEASE come home.