How do you cure a sneak?

Natalie is regressing. In the last month she has:
stolen 2 bottles of nail polish and dumped them (on my carpet. 'nuf said); she has taken red Sharpie to her face and shirt (Brady stole the pen, then left it out); she snuck into my scrapbooking stuff to take a jar of buttons (you know what happened there); she kept sneaking into my mom's guest room today to sneak Andes mints (even though she was told no more after my mom gave her 2); she covered her body with hand sanitizer today; she (and Riley) climbed on top of her dresser to get the fish food that was placed about 3 feet over MY head and dumped the entire jar of food, along with some knick-knacks into the fish tank, effectively suffocating several fish. The list goes on and on. And the longer I make it, the worse I look on paper, like I'm never around to monitor this.

But she is sneaking around to do this crap. So everything "appears" hunky-dory, but she is hatching a plan.

And at 3 years old, I thought my follow-the-baby-everywhere days were easing up. Nope.

She is, however, confirming that Terrible Two's are a myth. It's Three that might kill ya. Or her.

Any ideas on how to stop a sneaky vandal? She is so pathetic when I punish her, you have no idea. "I"m suh-suh-suh-sorry Mommy!" "I won't do it again!" "I wu-wuv you, mommy!" "You not angry at me anymore?" "See? I'm a good girl, now. Does that make you happy?" She works it, you try to keep a clear head when she starts flinging that at you.