Smart as simple. Good tip, Lance. Now, according to documentation, the AMap<class K, class T, class V, class HashFn>::FindAdd() method returns negative if the key is not found. I suppose that de docs are outdated because names a param that is no there:
int FindAdd(const K& k)
...
x Key to find.
h Precomputed hash value.
Return value Position of element or a negative value if element is not in AMap.