This seems like a good occasion to quote the twist reveal in Orson Scott Card's Dogwalker:

We stood there in his empty place, his shabby empty hovel that was ten times better than anywhere we ever lived, and Doggy says to me, real quiet, he says, "What was it? What did I do wrong? I thought I was like Hunt, I thought I never made a single mistake in this job. in this one job."

And that was it, right then I knew. Not a week before, not when it would do any good. Right then I finally knew it all, knew what Hunt had done. Jesse Hunt never made mistakes. But he was also so paranoid that he haired his bureau to see if the babysitter stole from him. So even though he would never accidentally enter the wrong P-word, he was just the kind who would do it on purpose. "He doublefingered every time," I says to Dog. "He's so damn careful he does his password wrong the first time every time, and then comes in on his second finger."

"So one time he comes in on the first try, so what?" He says this because he doesn't know computers like I do, being half-glass myself.

"The system knew the pattern, that's what. Jesse H. is so precise he never changed a bit, so when we came in on the first try, that set off alarms. It's my fault, Dog. I knew how crazy paranoidical he is, I knew that something was wrong, but not till this minute I didn't know what it was. I should have known it when I got his password, I should have known. I'm sorry, you never should have gotten me into this, I'm sorry, you should have listened to me when I told you something was wrong. I should have known, I'm sorry."