Ruin in the Cache
So, you'll notice that Ruin appears to Vin here in the form of Reen. One might wonder why he even needed her to investigate if he could visit the cache himself.
This reveals the main problem Ruin was dealing with in this instance. The Lord Ruler was very clever in how he placed and organized these caches. He planned them in locations where there was so much metal in the ground that it would prevent Ruin from discovering them. And, more importantly, he trained his obligators—Yomen included—not to speak of what was down below or reveal the locations of the caches.
Ruin didn't know there was a cache here, not until Vin found the previous plate. Even once he knew where the cache was, he couldn't see much when he visited it. He couldn't know if there was atium there, for the entire area—particularly because of the metal cans Vin mentions—glowed so brightly that Ruin had no idea what he was seeing.
He needed a pawn to visit, one through whose eyes he could see. One who could discover where the atium was. Ruin drew the same conclusion Vin did here—that if there had been atium, Yomen would have moved it. But where? Ruin still needed her to find it for him. Either that or bring in an Inquisitor, something he eventually decided to do.