When Catherine, the security guard and the police are reviewing the security tapes, Catherine asks for the guard to put the tape at the time of the crime.

The tape indicates a time of 5:48.

Thomas Crowne had looked at his watch when the "thiefs" were preparing to steal the paintings and claimed it was only "Quarter to five" - or about 4:45.

By 5:48 the temperature would have dropped far enough to see the contents of the room. The tape is also not off by one hour, because we see Crowne look at his watch in the morning before he puts the brief case under the bench and it is 9:10. When Catherine has the guard go to the morning, he shows the tape around 10:15. If the tape were off by an hour, there would have been no brief case under the bench, and no where near that many people.