To explain more fully, there are non-alpha characters, such as ‘-’, which are used as shortcuts for Boolean operators, such as NOT (which you discovered) as well as other non-alpha characters that are used as wildcards. Non-alpha characters, regardless if they are or are not assigned as a shortcut for a Boolean operator, are never searchable. Wildcards and Boolean operators are discussed in Search Operators section of the DEVONthink manual.
Is there is list of special characters to be avoided? I have a habit of typing  during meetings so that I can quickly spot something that should be turned into an actionable item in my todo system, or brought up later in a conversation. I generally process these after a meeting, and a search within the document works perfectly at finding . I never took the time to think about why the shift-command-F search didn’t work, too. I think I should change my habit to something that works in all cases, probably something like typing a three letters back to back, ooo, for instance. (Thinking out loud.)