Klamath County Sheriff's Detective John Dougherty refused to say what was written on the wall. Derrick's mother says it suggested her son may have been abducted from a snowy mountain against his will.
Wanting to pursue every possible lead in the case, authorities last week removed part of the wall from the bathroom on Oregon's high desert.
The section was sent to the state crime lab to check for fingerprints or anything else that might shed light on its origin. Dougherty said it was "Derrick-specific," though it did not name the boy.
Police said travelers at the Sagehen Rest Area about 290 miles southeast of Portland saw the graffiti Sept. 24 and called authorities.
On Saturday, Derrick's parents drove 250 miles from their home in the small farming community of Bonanza, near the California line, to the rest stop to have a look for themselves. They left unconvinced that the message was significant.
"I think it's just a big, sick joke," Lori Engebretson said Monday. "I thought, if somebody would have had Derrick, if they put this on the wall, they were wanting to be caught. If they were wanting to be caught, why didn't they leave something of Derrick's there?"
Not a trace of Derrick; not his gloves bearing the logo of the Goosebumps books he loved to read; not the jacket he hated to wear or the hatchet he carried. Nothing has turned up since he disappeared Dec. 5 on the east side of the Cascade Range.
"All I can say is I hope if Derrick did die, I hope he died on that mountain and not at the hands of some sick person," Mrs. Engebretson said. "If he died on the mountain, he just closed his eyes and went to sleep. There would have been no pain or anything."
The boy had been out with his father and grandfather. He had been sent up a hill with a hatchet to cut down a tree. The father and grandfather each thought the boy was with the other, and in a terrible moment they realized he was out there alone.
After Derrick disappeared, a snowstorm blew in. Helicopters with heat sensors, tracking dogs and hundreds of volunteers all combed the woods for more than a week.
After the official search was called off, Derrick's family spent nearly every weekend on the mountain looking for him. They have found someone's glasses, shoestrings and the tag out of a T-shirt, but no sign of Derrick.
The remote-controlled Big Foot truck and laser-sighted BB gun Mrs. Engebretson bought her son for Christmas are still wrapped and underneath the artificial tree in the family's home.
"There's clothing I can't imagine would fit him now, even if he does come home," she said. "The presents, the paper is starting to come off because it's so old."
"At first, it was tough, but now it's jusa permanent fixture in the house. Nobody says anything about it anymore. Every once in a while we just stare at it. But nobody talks about it anymore," she said.
"I haven't given up on him being alive. I pray every single night for him to be alive in somebody's safe hands."