Sometimes you will see a "None" group in the monitor status page, like so:
This is usually due to a host's hostname being changed, and will age out after 24 hours.
Additionally, when a monitor is changed, until the next evaluation, it won't have any groups (e.g. a host, set of hosts with a single tag, etc.) for the new monitor query, but will continue showing the old groups for 24 hours. This can also result in "None" groups.