Netcap writes a general summary to stdout, if you wish to disable output entirely use the -quiet flag:
$ net capture -quiet
When the quiet mode is used, the output is instead written into the netcap.log file in the directory where netcap is executed from.
Errors when parsing packets are logged by default into the errors.log file in the current directory.
Each log entry contains a hex dump of the entire packet and the error message or stack trace.
The following log file are produced when running with the -debug flag:
debug.log: general debug messages
reassembly.log: tcp stream reassembly debug logs