A quick peek at the logging code shows it will say "pass through" for a number of reasons, not only a failed match.
Processing will be stopped immediately if the NS (ignore on subrequest) or R (force redirect) flags are encountered; P (force proxy) and L (last rule) can also cause it (although I'm not sure of the exact circumstances), and so does using C (chain) on the last rule in a set. A failed RewriteCond will also trigger it.
As far as I can see, none of these condition will cause any other log messages to be written.