I have run into a few people at work over the years who "deny, deny, deny." For some reason they think it's like a courtroom where if you are not convicted then it's like it never happened and there is no punishment.
Except it doesn't matter if there is proof, everyone knows they are guilty and at the first opportune moment they end up unemployed. What amazes me is they think if nobody has concrete proof and they don't admit it, nothing can happen to them.
And even if nobody makes an accusation, it doesn't mean they don't know what happened or that nothing is going to be done.
One guy was hired as the director of marketing probably making 6 figures, with 20+ years of experience, and didn't make it 3 months. First incident was he traveled to Chicago to interview a bunch of different people to fill a marketing job - he only interviewed 3 people in a week and said there were a bunch of no-shows and some people who cancelled for various reasons. Second incident was when he was the only person from our company attending a particular trade show and on the second day he was only there for about an hour.
In both cases they found out (innocently, not that they were checking up on him). He was never confronted about it. Once they found out, they just got the ball rolling with HR to get him fired. He probably still doesn't know exactly why he was fired.
Liars never think anyone is smart enough to catch them. But keep in mind that in the workplace there isn't always a need to "catch" someone in a lie.
A reputation for integrity is very difficult to regain once it's lost.