This is because of a limitation in Confluence. We've filed a request with Atlassian.
If Gliffy Plugin for Confluence customers are concerned about this issue, PLEASE vote for CONF-9235 at Atlassian to raise the priority:
http://jira.atlassian.com/browse/CONF-9235
The basic issue is that there isn't a mechanism in Confluence which will allow us to send search results for Gliffy attachments to Gliffy defined pages (ie the Gliffy Editor or full screen view of a diagram).