The sessionid returned from userenv('sessioid') is not listed in the output of V$SESSION.

SQL> select userenv('sessionid') from dual;

USERENV('SESSIONID')
--------------------
18817

SQL> select username,sid,serial# from v$session;

USERNAME SID SERIAL#
------------------------------ ---------- ----------
1 1
2 1
3 1
4 1
5 1
6 1
7 1
PFIN 9 3284
SPA 10 1490
SPA 16 2298
SPA 18 788

USERNAME SID SERIAL#
------------------------------ ---------- ----------
PROSPECTOR 19 975
21 957
SPA 22 480

14 rows selected.