Listener listening without listener.ora

I have an Oracle DB working in production for more than a year.
I have my listener working:

[oracle@base admin]$ lsnrctl status

LSNRCTL for Linux: Version 11.2.0.4.0     
Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 11.2.0.4.0 - Production
Start Date                02-NOV-2016 14:22:38
Uptime                    1 days 20 hr. 3 min. 13 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /o/app/oracle/diag/tnslsnr/base/listener/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=base)(PORT=1521)))
Services Summary...
Service "BASE" has 1 instance(s).
  Instance "BASE", status READY, has 1 handler(s) for this service...
Service "BASEXDB" has 1 instance(s).
  Instance "BASE", status READY, has 1 handler(s) for this service...
The command completed successfully

But I have no listener.ora:

ls /o/app/oracle/product/DB_11.2_SE/dbhome_2/network/admin/
samples  shrept.lst  tnsnames.ora

This is the content of my tnsnames.ora:

$ cat tnsnames.ora 
BASE =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 10.20.20.19)(PORT = 1521))
      (LOAD_BALANCE = yes)
    )
    (CONNECT_DATA =
      (SERVICE_NAME = BASE)
    )
  )


ORCL =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 10.20.20.20)(PORT = 1522))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = orcl)
    )
  )

How is this possible? As far as I know, for the listener to work, I have to have a listener.ora file.

I have another Oracle database installed, and if I execute lsnrctl status, in the output I can see a line containing Listener Parameter File /o/app/oracle/product/11.2/dbhome/network/admin/listener.ora, and I am not seeing it here. Also I have executed locate listener.ora, and nothing.