Where does forever store console.log output?

Forever takes command line options for output:

-l  LOGFILE      Logs the forever output to LOGFILE
-o  OUTFILE      Logs stdout from child script to OUTFILE
-e  ERRFILE      Logs stderr from child script to ERRFILE

For example:

forever start -o out.log -e err.log my-script.js

See here for more info