MISSING ROWS IN AUDIT TABLE "SYS.AUD$" FOR LOGON EVENT (code 100) USING OS AUTHENTICATION (OPS$ USER

HI, EVERYBODY! RECENTLY I HAVE ACTIVATED AUDIT OVER AN ORACLE DATABASE 9i AND I'M EXPERIMENTING A SUSPICIOUS BEHAVIOR WITH O.S. AUTHENTICATION.
NORMALLY, WHEN IT PRODUCE A LOGON IN THE DATABASE (AND IF THE USER HAVE ACTIVATED AUDIT ON CREATE SESSION), IS INSERTED INTO THE TABLE  "SYS.AUD$" A RECORD WITH A CODE "100" THAT IT BELONG TO "LOGON" EVENT.
IN THE CASE OF USERS AUTHENTICATED ON "O.S." (OPS$ USERS), I DON'T SEE ANY RECORDS WITH AUDIT CODE "100" , ONLY I CAN SEE RECORDS WITH AUDIT EVENT CODE 101 ("LOGOFF" EVENT).
THAT'S CORRECT??  IS THE EXPECTED BEHAVIOR WHEN THE DATABASE IS ACCEDED WITH "OS AUTHENTICATION"?