OK...Who Did It?!?!?!?

Soooooo....my Sheltie named DAISY was at her cousin's house today for a doggy playdate.  She too a Sheltie named TRINNY.  It's been an ongoing challenge the past few months where as soon as they we're left alone, one of them would pull open the treat drawer and devour a COSTCO size bag of chicken treats.  At first we we're slightly entertained by it, trying to imagine which of the two had the smarts to pull open the drawer, which is kinda heavy.  

Then after a handful of times, and me trying to trim Daisy's weight down...not so funny.  
(PLUS it would also mean several outings in the middle of the night, if you catch my drift:).

So, we decided to put a kid drawer-catch/lock on it.  

That worked the first coupla times, as it would catch the drawer above it and they would not be able to reach the treats. Well, note what I just discovered when I went to pick Daisy up.  Really?!?!  How did they figure that out?!?!?  And which one is doing it?

Daisy (top), Trinny

Hmmmmmm.  Now what?  Please advise friends:)