PL/SQL :: When Oracle Invalidates Result_cache Results Without Any Changes In Objects

On our production servers we have simple function with result_cache, like this:

create or replace function f_rc(p_id number) return number result_cache
is
ret number;
begin
select t.val into ret from rc_table t where t.id=p_id;
return ret;
exception
when no_data_found then
return null;
end;

/And its results frequently invalidates without any changes in table or function. I found only 2 cases when oracle invalidates result_cache results without any changes in table:

1. "select for update" from this table with commit;
2. deletion of unrelated rows from parent table if there is unindexed foreign key with "on delete cascade".

I test it on 11.2.0.1, 11.2.0.3, on solaris x64 and windows. Test cases:

[URL]......

But none of them can be the cause of our situation: we have no unindexed fk, and even if i lock all rows with "select for update", it still does not stop invalidating. In what other cases this happens? Am I right that the oracle does not track any changes, but the captures of the locks and "commits"?