I just located an invisible chest that lies literally inches from the Khajiit merchant Ma'dran when his tent is set up outside Solitude. Sadly, I dont believe it is his. I tried to do the same "loot-kill-loot-kill" routine that can be done with the Dawnstar merchant, but when I tried it for the first few times it didnt restock or change the contents of the chest in the ground. In fact, his gold was 792 when the gold in the chest near him only had 750 just like the Dawnstar merchant. So it cant be his chest, even though the items resemble the stock these merchants carry (Soul Gems, Potions-a-plenty, similar gear, Ingots) I dont think it would be a duplicate of the Dawnstar chest though, that wouldnt make sense... But its something I'll check out eventually.
I am currently trying to find the other chest near Ri'saad's location outside of Markarth. Apparently it can be found by searching on or near a wall behind/around his camp site on the side nearest the river. No luck yet, but I dont even know if Im searching the correct wall(s).
My theory is that the creators may have thought we'd find these chests so they seperated Ri'saalds and Ma'drans chests to be on opposite sides of the world from their respective owners to avoid exploitation. This would really piss me off, considering the fact that the chest near Ma'dran is so close to him I could swing to kill AND loot the chest without taking a single step vs the long walk to and from the Dawnstar merchant and her chest. Took me a few hours to find the one near Ma'dran.
I'll repost if I can find Ri'saads chest and possibly prove this to be true. Otherwise i'd love more insight on this as well from anyone else familiar with these merchants and their hidden stuffs.
EDIT:
Yup... I was right. I didnt locate the "wall chest" yet, but the contents of the invisible chest near Ma'dran belong to Ri'saad. I looted the chest, found Ri'saad at his camp near Markarth, browsed his stock and he had 0 gold and no soul gems. I saved, killed him, reloaded, checked his store to make sure he had money, fast traveled back to Solitude to check the chest and it was restocked from the save-kill routine with the same amount of money Ri'saad was just carrying. No doubt now that Ma'drans chest is the one hidden near Ri'saads camp as well. What a pity.