will send output to where ever WeeWX log output is sent, /var/log/syslog is typical for Debian based users unless you have explicitly changed it.
However, note that unknown and unmapped sensor data is only logged when the sdr driver is run in conjunction with WeeWX (ie the driver's genLoopPackets() method is called). Unknown and unmapped sensor data (as controlled by log_unknown_sensors and log_unmapped_sensors) is not produced when running the sdr driver directly.