If you have a dedicated maintenance page (eg. maintenance.html), we'll automatically pick that up.
If you display a maintenance message on an existing page URL structure, use Action Properties to grab the selector that displays the maintenance message. Then you'll be able to use the RUM multi-dimensional analysis to select that property from the Analyze By dropdown to see what pages it is occurring on. You could even create a custom metric to get alerts if the number of maintenance pages being displayed goes beyond a set threshold.
