Quite simply, Tom's and Daisy's disappearance illustrates their total disregard for anyone other than themselves; therefore, their escape can be directly tied to the theme of corruption.
I called up Daisy half an hour after we found him, called her instinctively and without hesitation. But she and Tom had gone away early that afternoon, and taken baggage with them.
"Left no address?"
"Say when they'd be back?"
Is there any better example of moral corruption than a rich gal happily and easily running from the law with her husband while making an obsequious lover the fall guy? Thank God for money, eh Daisy? Adding to the evidence of corruption here (and adding to the contrast) is the fact that our innocent little, Midwestern narrator, Nick, originally thinks that Daisy is going to be as concerned for Gatsby's poor soul as Nick is. You called her "instinctively and without hesitation"? Did you really think she was going to pick up the phone? Poor Nick.
In my opinion, Tom's and Daisy's escape also proves that Daisy married Tom for security. She desperately needs security at this point in the novel. She's a murderer, for goodness sake. She killed Myrtle in a hit-and-run accident and then let Gatsby face the music. Tom and Daisy are escaping (and with their money and with Gatsby as a scapegoat, they can).