Anyone know where this guy is hanging out?

Are you talking about the merchant near the harbor that is marked on the minimap but you can't get to him because he is apparently in a locked house?

If so, I would also like to know what's up with that.

No I mean the DLC merchant.

Actually, we both meant the same merchant - but I didn't know I was talking abou the Mysterious Merchant.

I was also wrong about him being inside the house.
The Mysterious Merchant DOES sit where the minmap indicates, but that is outside under this balcony where all the bundles and barrels are.

However, if you don't have the Mysterious Merchant DLC there is still an merchant icon on your minimap where he would be - at least I had this happen to me.
First I though the merchant was supposed to be inside the house, which is always locked and you can't enter at any point in the game ( at least AFAIK).

Apparently there are mods that add merchandise to "The Illusive Man" and we have to wait until CD Project's agreement with gamestop expires before we have opportunity to use this cheat!!