Alright here you go, Detective Tissue was inspect-ing a scene. A woman, who by many was thought to be eccentric, lay dead in the cave. She kept an animal as a "watchdog" of sorts. Only this animal was, in fact, a duck. She was out to be killed by an unknown man, so she hid in the caves. Not even she knew who was trying to kill her. The duck was well-trained enough to quack if an intruder came nearby, and the thought was, that the sound would echo enough for the woman to hear the duck. The woman was not close enough to the duck to be able to hear it without the echo.
A man was at the crime scene, and he said that he saw someone go inside the cave.
"Well," the man said, "I knew this woman was hiding in these caves. I own these lands, and I allowed her safety. Well, I check up on the duck every now and then, and feed it. I also bring the woman some food, and the duck doesn't quack at me anymore. The woman told me that would happen eventually, and I'm quite proud I'm the only person who is like that besides her. You know, when the duck's echo rings through the walls of the cave, her plan was to escape through the other side. The other side is near a river, and no one can get through it except from within. Special gate I made myself. Anyway, I then realized that the duck's quack doesn't echo. So, I was planning to tell her this, when I see a man walking from within the cave. He was holding a bloody knife, which he dropped when he saw me. He ran away before I could stop him. Poor woman. If only the duck's quack had echoed, she would've had more than enough time to get through the gate."
When Tissue inspected the gate, and found it not to be jammed, he promptly placed the man under arrest. Why?