@DrChirper - Sigil 0.5.3 is now ancient history, any comments about behaviour should be made using the Sigil 0.5.9 betas (a new version will be out in the next few days).
The reason the Find has to switch to Code View is because it simply isn't possible to search in Book View reliably (at least not without opening every file searched in a new tab, which is both slow and annoying). And it most certainly isn't possible to replace reliably. Hence why all replace operations must be in Code View, and the only Find operations that will work in Book View are for current file.
As for the F&R expression, just change the Mode dropdown to choose "Case Sensitive" rather than "Normal". Or if you were using Regex, that too is effectively case sensitive.