Dashboard searches (and other searches like this) always run in fast mode. So if you click the little magnifying glass to "break out" a search from a dashboard then it will carry-over that setting to your search pane. This is probably whats happening to you.
If you do set Smart/verbose mode, it does get saved in your personal settings directory, but only on a PER-APP basis. I expect this, coupled with the dashboard searches I mentioned above, means you will find yourself changing the search mode setting regularly.
I don't believe there is any way to change this behaviour.
All the best.
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