Steps to reproduce:

1. Have 2 host groups (Host group A and Host group B) setup with admin users and hosts but where the admin user from Host group A only has permissions to see Host group A and admin user from Host group B only has permissions to see Host group B.
2. Log in to Host group A with the admin user and create a map (Monitor > Maps).
3. Log out and log back in using the admin user for the Host group B and verify that you cannot see the Host group A map (Monitoring > Maps) as expected.
4. Log out and log back in using the Host group A Admin user and add an image to the map (Element with no host associated to it).
5. Log out and log in as a Host group B Admin and observe that you can now see the group A map which is not expected (Monitoring > Maps).

Result:

Host group B user can Host group A map when they should not be able to. It looks like if a map element is not associated with a host that this bleedover occurs. Simply associating an element with a host or removing the element the Host group B users can no longer see Host group A maps. 

Related Jira https://support.zabbix.com/browse/ZBX-22302 

Also note that setting sharing so only group A can see the map does not fix the issue. 

Expected:
Expecting that Host group B should never see Host group A maps when an element that is not associated to a host is on the map like a label.