SYSDATE is also not a column in DUAL table, yet you've probably often used "SELECT SYSDATE FROM DUAL;". The same goes for UID, for example.
It is nothing misterious about USER, SYSDATE etc. They are normal built-in functions. The only thing that differs them from other built-in functions is that they require no parameters.
And they are all documented in SQL manual, so there is no need to look for them among "undocumented features"...
ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?