When a MongoDB server runs as a daemon (i.e. with --fork) and is supplied a relative path for the the setParameter trafficRecordingDirectory, that parameter's validator (validateTrafficRecordDestination() in
traffic_recorder_validators.cpp) calls boost::filesystem::is_directory() on the raw string while
the option is being parsed, (which happens before the fork, so against the launcher's current working directory) but after the server forks itself and changes its working directory to "/", the recorder opens that same string but now relative to the daemon's cwd of "/".