thought not/thought so

Hello,

I am reading "Superfreakonomics" by Levitt and Dubner and encountered the above mentioned expressions:

For human beings, however, scrutiny has a powerful effect. Do you run a red light when there's a police car--or, increasingly these days, a mounted camera--at the intersection? Thought not. Are you more likely to wash your hands in the office restroom if your boss is already washing hers? Thought so.

I would like to analyse these "Though not" and "Thought so" from the grammatical point of view. Which subject is assumed here? 'We', 'I', 'one', 'they'? Why is the past tense used here? How have these expressions come into being and are there any ones similar to these?