How do I ensure my corn snake enclosure is escape proof?

I keep my corn snake in my bedroom on top of a chest of drawers & I took her out 2 handle her @ about 8.00pm this evening. I went 2 say goodnight & check on her @ about 10.15pm but she wasn't there! I checked her hides & under the substrate but she was definately gone! By shear coincidence & blind luck I happened 2 look down & there she was draped over the side of my sock drawer. She hadn't gone far thank goodness but how did she escape? I hav an enclosure with sliding glass doors with a lock on the outside. It has an interior heating mat, so had 2 small holes either side of the enclosure 2 put the cables 4 the heat mat thru. We blocked off the hole th@ was not in use with sum cardboard & tape & the other was, I thought 2 small 2 get thru with the cables goin thru. We hav now put sum tape over this hole (with the cables in) aswell 2 b safer but wat else can I do? How did she get out? We hav a cat & a dog which we keep downstairs away from the snake enclosure but if she escapes again & the dog or cat get 2 her 1st...I dnt wanna even think about it! Please help!