i'm not a great C programmer, but i think i understand, from the comments on the method which creates the mutex, that an actual file is used only if the operating system needs it to implement the locking.
correct
your Linux build is probably defaulting to SysV semaphores*, and mod_rewrite uses the default mutex type; thus, there is no file associated with that mod_rewrite mutex
*verify by seeing if apachectl -V displays "-D APR_USE_SYSVSEM_SERIALIZE"
