If this isn’t a boundary you’ve styled and made permanently visible via the eye icon, then this indicates a title is 'open', or currently selected from within the title panel.
A title is 'open' or selected if it appears larger, with additional address and/or ownership information and gives you the option to press the 'Open Title' button.
To remove the red boundary you need to close or deselect the title. To do this, click on the title within the title panel on the left-hand side.
