Every dog owner knows the telltale look of a dog who did something it wasn't supposed to do.
You know she did something she shouldn't have done and, seemingly, she does too. Since you're a human being, you see that look and ascribe a common human emotion to it: guilt.
All the logic lines up: Your dog was left alone, did something they weren't supposed to do (that they know better than to do), and when they're called on it, their face says it all. Perhaps you're already saying "No! Bad dog! Bad dog!" or some variation thereof.
The truth is, despite your logical summation, the dog isn't feeling guilt. Instead, they're expressing a much more common, less complex emotion: fear.
Don't just take my word for it: That assertion is based on a 2009 study conducted by dog cognition scientist Dr. Alexandra Horowitz, author of 2009's "Inside of a Dog: What Dogs See, Smell, and Know" and 2016's "Being a Dog: Following the Dog Into a World of Smell."
Dr. Horowitz's 2009 study, "Disambiguating the 'guilty look': salient prompts to a familiar dog behavior," specifically focuses on the concept of how humans interpret dog emotions through the scope of human emotion. More simply: Humans tend to misattribute dog emotions based on human emotions. The "guilty" look is a prime example of this.