I tried to search the forums and could not find the answer. What do the ref circles mean around obstacles on the charts?
I believe they're circled if they're vertically close (i.e. you're low).
What is said above is true, this means you have obstructions turned on in the map mode settings. If you click on the gear button next to obstructions in the map mode tab, then you can change various thresholds on these alerts.