This post explains how Oracle Net searches for the configuration files listener.ora, sqlnet.ora, tnsnames.ora and cman.ora. Oracle Net Services configuration files are not always located in the $ORACLE_HOME/network/admin directory. Depending on the file, Oracle Net uses a different search order to locate the file.
Search order for the sqlnet.ora
1. The directory specified by the TNS_ADMIN environment variable, if set.
2. The $ORACLE_HOME/network/admin directory.
3. In RAC, the sqlnet.ora file referenced by the instance would be in RDBMS_HOME/network/admin and NOT the GRID_HOME/network/admin.
Search order for the cman.ora, listener.ora, and tnsnames.ora
1. The directory specified by the TNS_ADMIN environment variable, if set.
2. One of the following directories:
On Solaris: /var/opt/oracle
On other platforms: /etc
3. The $ORACLE_HOME/network/admin directory.