Mr. Brooks

Earl Brooks (Kevin Costner) is a wealthy, successful businessman recently honored by the Portland, Oregon Chamber of Commerce as "Man of the Year". However, Brooks lives a secret life as a serial killer, known as the "Thumbprint Killer". Brooks has abstained from murder for the past two years by attending twelve-step meetings for addicts, to try and cope with his "killing addiction". He feels the compulsion to kill rising again, however, as his id, Marshall (William Hurt), becomes more insistent. Brooks gives in to his addiction and kills a young couple while they are having sex in their house and, as part of his psychopathology, leaves each of the victims' bloody thumbprints on a lampshade. Brooks follows his meticulous modus operandi, including fastidious preparation, cleaning up the crime scene, even locking the doors before departing, and burning the clothes he was wearing during the killings. Marshall then realizes that the couple's curtains were open, facing an apartment building.
Brooks' daughter Jane (Danielle Panabaker) unexpectedly arrives home, having dropped out of college in Palo Alto, California. She visits Brooks at work and mentions that she would like to get a job with his company. The same day, a man calling himself "Mister Smith" (Dane Cook) turns up at Brooks' work and blackmails him with photographs of Brooks at the most recent murder. Smith demands that Brooks take him along on a murder, to which Brooks reluctantly agrees.
Brooks' wife, Emma (Marg Helgenberger), reveals that Jane dropped out of college because she is pregnant. The Brookses are then visited by detectives from Palo Alto who want to interview Jane about a murder committed in her former dorm building. Marshall and Brooks realize that Jane committed the murder and consider letting her go to jail to "save her" from becoming like them. Eventually, however, Brooks uses an alternate identity, flies to Palo Alto, and commits a similar murder to make it appear as if a serial killer is loose, thereby exonerating Jane.
Brooks researches the background of the police officer chasing the Thumbprint Killer, Detective Tracy Atwood (Demi Moore), and discovers she's in the middle of an agonizing divorce from Jesse Vialo (Jason Lewis). Brooks decides that Vialo and his lawyer, Sheila (Reiko Aylesworth), will be Smith's first "victims". At the scene of the Vialo murder, Smith wets his pants in a fit of panic, leaving his DNA for the police to discover later.
While driving away from the scene, Smith pulls a gun on Brooks, which Brooks and Marshall had predicted would happen. Brooks explains to Smith that he knows he can't stop killing, and so plans to commit suicide to spare his family the shame of his being caught. Brooks takes Smith to a cemetery he owns, and explains that they will find an open grave. Smith will shoot Brooks and then cover him with just enough dirt to mask the body. The next day, a casket will be lowered into the grave and covered, and Brooks' body will never be discovered.
Smith attempts to shoot Brooks, but Brooks reveals that, at some point prior, he had broken into Smith's apartment and bent the firing pin on Smith's pistol, rendering it inoperable on the off-chance that Brooks would change his mind. Brooks' brush with death makes him realize he wants to live to see his grandchild, and he turns on his would-be murderer, slitting Smith's throat with a shovel and hiding his body in the open grave. With Smith's urine providing the only DNA evidence of the Thumbprint Killer at a murder scene, Brooks will remain undetected. Smith is named as the Thumbprint Killer and Brooks returns to his normal life.
Knowing he is in the clear, Brooks calls Detective Atwood, whom he has come to admire, to ask her why she is a police officer. She replies that her wealthy father had wanted a boy, and she wanted to succeed in spite of him. Atwood is unable to trace the call before Brooks hangs up, but she is puzzled as she realises Smith was not the voice on the line and that they may have fingered the wrong guy. That night, Brooks has a nightmare in which Jane murders him, suggesting that he fears Jane will become like him.

Question: What is the only evidence tying the Thumbprint Killer to the scene?
Answer: Smith's urine.