The police were checking the place. Outside, Mary, Mr. Olson's faithful secretary, was a mass of nerves. Inspector Will was reviewing the facts:
1. Mr. Olson has been murdered. The murderer buried a stiletto through the back of the chair and right through his heart.
2. All the place was a mess, but nothing seems to be lost. Some sheets on his desk had coffee stains, but Mr. Olson was not a coffee drinker. There was no coffee container of any kind in the office. There were a couple of gloves in the floor.
3. Mary had in her visit book just three announcements: Hari Colton (2:35), Louis Williams (3:10) and Joseph Markis (3:45). Mary says that Joseph was the only one to serve himself a cup of coffee in a paper cup.
4. The cup of coffee was found in Mary's wastebasket. She claims that Joseph entered Mr. Olson's office with the cup in his hands, and then he left it in her desk, so she took it and threw it away.
5. The stiletto had no fingerprints at all, but the paper cup was all covered with one type of fingerprint: Joseph's (they had a mobile fingerprint lab).
6. Mr. Olson's wristwatch was broken, probably due to the fight, and it showed the time at 3:50.

Inspector Will then talked to Mary, to tell her she was under arrest.
Why did he suspect her?