the red box refers to other dataframes but it's only showing over land and not over the ocea, ie, the box is an incomplete square
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It looks like you have used a Clip option on the Data Frame tab of your Data Frame properties for the data frame that you are using as your overview.
The ArcGIS Online Help describes Clipping the Data Frame.
Change its current setting to No Clipping and I think you will see your Extent Indicator in its entirety.