Dont think there are any. Ive gone through all systems inspecting the ground as far as player can go and found nothing. It might be that there is some hidden extremely well.

Mandalore and Kamino have one, but I do not know of any others.

where are they?

On mandalore, you can find Boba Fett's Rifle & Armour; its on the far back left (from the direction you start in) platform, you have to jump down to it from a higher platform, and you should see a bronw/greenish crate that you can open

There is already an other discussion about secret weapons and amor:
http://psteamcommunity.yuanyoumao.com/workshop/filedetails/discussion/742671195/343785574522001477/

I think, the weapons and armor, which are mentioned in this discussion are the only one, which are implemented at the moment, but I don't know, if you have to use that kind of cheat, which is explained there or if you can find all the chests without it.

You have to use the method I described at the moment