If --pidfilepath is specified on the mongod or mongos command line and the path to the pid file does not work for any reason, no pid file is written and no error message is displayed.

Probably, this should cause a EXIT_BADOPTIONS exit similar to what would happen with any other unusable file specification on the command line (dbpath, logpath, config).

./mongod --pidfilepath /there/is/no/such/path/or/file
// normal startup, no file written, no error message