Mornin'.
Detection is one of several skills that feed into investigation. If you are actively attempting to detect a thing, you are also investigating. But when you are actively investigating a thing, you are not always detecting. You could be engaging a different related skill, or often several at once.
It works the same way intelligence does. They have skill sets full of more active and obvious words that when in use for the specific purpose of investigating or intelligencing both the set skill and the parent skill should be awarded with appropriate lore.
Now if you really want to be confused, you can investigate a thing as an act of intelligence and you can also use intelligence techniques in an investigation.
I hope this helps!