When you open a 2D file that has been marked up in A360 Viewer, all markups for that version of the file will be visible. You can show or hide these markups by clicking Hide/Show Markers within the Comment & Markup Utilities. To view a specific markup and related comments, open the Comments Panel and click the comment for that markup. All other markups will be dimmed. To view all markups again, click the selected comment again.
When you click Markup to add markups, all existing markups are visible in a dimmed state and you can add markups over existing markups.