This isn't just Outlook, it's a ZWC 5.0.6 thing as well.
What happens is that the meeting organizer fails to check the Schedule tab before setting up their meeting and double books the resource.
The resource correctly declines the meeting, and this appears on the resource's own calendar. On the organizer's calendar, however, there is no obvious indication that the resource was not available. The only way to tell is (again) to go to the Schedule tab and notice the unhappy red 'X' next to the resource's name.
This should probably turn into an enhancement request that, in the case where a Location has declined your meeting, the Location name should be marked with a red X in both the informational pop-up and the main 'Details' tab.
The only other option is training your users to always check and double-check the Schedule tab, but have you tried training users?
