Yesterday I saw this sentence in some test without any context: I ... (see) him a few days before he ... (leave). My answer was "I'd seen him a few days before he left", but it was wrong. Right answer was "I saw him a few days before he left". I understand that the second answer is also correct, but I don't think I'm wrong, or am I?
I saw him a few days before he left.
In this example without additional context, it is understood that you "saw him" on one occasion, a few days ago. Therefore, past tense is sufficient.
I had seen him a few times before he left.
Here past perfect is OK because you "saw him" over a period of time in the past.
The PAST PERFECT TENSE indicates that an action was completed at some point in the past before something else happened.
For example : "The patient had died before the doctor came", let's understand this using a practical example, let's say the patient died at 5 PM then the doctor would have come after 5 PM.
So in your example if you say " I had seen him a few days before he left", this means that you saw him before he even left which is incorrect, that's why the correct answer was "I saw him a few days before he left".