I often have the problem when I flag something as duplicate and try to search in this searchbox I get no hits. But when I use the same keywords in my google search I find what I'm after.
With search box I mean
Yeah.. I usually flag as dupe from prior knowledge, and then use the main search or search engine to get the results.
There's also this thread with common canonical questions, which can help.
But you're right, the search box in flag as dupe is rather useless - I don't know if anything can be done about it.