Cozzi: “When the LOOKUP operation is successful, the index variable is set to the index of the located variable”. p 312
In other words it will not change on an unsuccessful LOOKUP. You can use NOT %Found to test for a failure to find correct index.:
Category LOOKUP ItemCat
if NOT %Found()
I’m not sure, but I believe that if the lookup is unsuccessful, the array index remains whatever it was before the lookup operation.