In Ops Center, you can place assets into maintenance mode to temporarily stop alerts from being generated on them. This is useful if you're doing maintenance on some systems, hence the name; but particularly in a large environment you can lose track of what assets are in maintenance mode.
There are a couple of ways to tell. For a single asset, if you've selected it, the Actions panel will have a 'remove from maintenance mode' option if it's already in maintenance mode.
If you want to see all of the assets in maintenance mode, you can create a group that will automatically include them. Create a group with a single group rule like this:
This new group will then include all of your assets in maintenance mode.