I agree - 30 mins is waaaay too little time to be able to tell anything about a Corn Snake. Some I've had for 4 years and I'm still trying to figure them out!
I'm guessing that the key will be whether your Corn was digesting or not - a full stomach would probably have prompted a fairly quick trip to the warm end, although not all Corns will stay put there. Some will go there for a few minutes, then go and cool down, then go back etc.
There are other factors to consider as well which might influence how a Corn behaves. How the tank was laid out, for example. Did you have hides at both the cool and warm end? Were they both the same or were they different? If the snake was being observed, was there movement or noise outside the tank for the 30 minutes? This might have made it wary and want to hide or keep still as a defence mechanism. Where was the water bowl?
It's an interesting idea, but I'd be more inclined to go for a 30 DAY observation period to give a more accurate picture.