I found this key called Small key from Hlaalo Manor.

So my question is does anyone have any idea what it unlocks?

My absolute first guess would be that it opens something in Hlaalo Manor.

But then again, it could be another one of those random keys that mods always seem to accidently add into the game.

Well there are only two locked doors in Hlaalo manor, the front door and the side entrance upstairs. I have a feeling that it doesn't open either but as I have always unlocked one or other door to get in possibly I am wrong. There are several unnecessary keys in the game.

In Caldera you will perhaps - if you search - discover a key called Abebaal slave key. It doesn't free the Abebaal slaves. Etc.

It's not a random key because it can be found on the floor in the upstairs.

I know. It is half falling out of a pot iirc. In the CS it call's it Ralen Hlaalo's key so I assume it must be to his house. And yet I can't recall being able to use it there. Possibly it was intended to have more to the quest at one stage and the key was going to be needed but then wasn't. Why don't you start a new game (or go back to an earlier save), use coc to get onto the house (if you can) take the key and use an intervention spell to get out. Then try it on the two house locks and tell us if it works on either or both.

It didn't work on the doors and I think that the guy who murdered was looking for it as the upstairs is in such a mess.