As per the previous two replies standard out and standard err logs are going to be generated by the application server.
I'm not sure which version of Pega you are using here, but if it's 7.4 or latter then we perform most logging through log4j. This can be configured in the prweb/WEB-INF/classes/prlog4j2.xml (or prlogging in earlier versions).
In the OOTB file this includes a logger/appender which will also send messages to standard out. Hence in this instance your application is sending these to the System.out file.