I may be hazy on some details, but I believe Columbo had a hunch that Col. Rumford got up “at dawn’s early light” and sabotaged the cannon. But there were no witnesses who saw him there, in fact, the “boodle boy” vouched that Rumford was in bed as expected.
When Columbo learned that the cider was discovered on Founder’s Day he wondered how and when Rumford made his discovery. Columbo hid the cider for the cadets and, in return, asked them where they had been hiding it. It turned out that the particular window they were hanging it from was hidden, except from the area out by the cannon.
Because Rumford had already admitted that he had seen the cider from a window, Columbo now had proof that Rumford had been standing out by the cannon early on the morning of the murder.
And further to the earlier reply, Columbo was aware of the Colonel's problem with the Cider when the Colonel yelled at the Captain in the dining hall, in front of Columbo, for not progressing the Cider investigation fast enough.
Nobody in the entire series says certain lines the way that Patrick mcGoohan does. Several years ago I put up a post of some of my favorite mcGooohan quotes. I should try and see if I can resurrect that post somehow but it was a long time ago.