When my mavic is operating the image shown on the app shows a red outline around the objects in the viewfinder. I've looked everywhere but can't find how to turn it off.

Does anyone know?

in aircraft settings 3 small dots and in there is Peak Focus Threshold , put it to "none".....and red will vanished

It does have a purpose those red outlines.
The red outlines tells you things things are in focus.
Very usefull when flying on bright sunny days.