On Valentine's Day morning, David Wandtke told his wife of 11 years he was going to the grocery store to buy junk food, a familiar errand for him and, his wife thought, possibly a cover for a last-minute gift run.

Wandtke, a 52-year-old nurse, drove away in a maroon PT Cruiser and hasn't been heard from since, becoming the second St. Paul man this year to apparently walk away from his life and into oblivion. "Right now we're treating it as an older man who decided to walk away from his family," said Sgt. Paul Paulos, a St. Paul police spokesman.

About 10,000 people are reported missing in Minnesota every year. Many turn up soon, their disappearances simply a misunderstanding, but others are abducted or led astray by mental health problems.

Some seem to have chosen to disappear. That category of missing persons can rock loved ones and confound law enforcement officials. In FBI missing persons records, they're lumped into the ambiguous category of "other," semantics that underscore how inexplicable such cases can be. The five remaining categories -- juvenile, endangered, involuntary, disability and catastrophe victim -- are absolute by contrast.

Why did Wandtke disappear? "We really don't have any idea at all," said his stepdaughter, Sara Pepin. "That's why it was such a shock. All we know is that he wasn't happy at work and he was thinking of finding another job."

Of all the people reported missing each year in Minnesota, about 500 cases remain active by year's end, a cumulative number that includes active cases dating to 1975 when reports were first tracked.

Last year, about 678,860 people were entered into the FBI National Crime Information Center's Missing Persons File. As of Jan. 1 of this year, 85,158 cases remain unsolved, also cumulative since 1975.

It's the exception --not the rule -- that a missing person stays missing.

Wandtke took his wallet, driver's license and a personal phone book, Paulos said. His snow-dusted car was found March 5 at Fort Snelling. It's the last clue to his whereabouts.

"You have to ask yourself: What law are they breaking? They're breaking our moral code, the marital code," Paulos said. "He didn't break any laws. We just want to make sure he's OK."

Police are taking the investigation seriously, he said. The passing of Brandon's Law in 2009 requires law enforcement in Minnesota to act immediately on missing persons reports involving adults. But in much of the country, the absence of similar statutes means the philosophy on missing adults can work against families.

Right to privacy

"In the United States you have the right to go missing if you want to," said Bill Carter, an FBI spokesman. "Individuals have the right to privacy."

The law is named for Brandon Swanson, who was 19 when he went missing near Tauton, Minn., in 2008. He called his parents after his car ran into a ditch. His phone suddenly cut off. His car was recovered, but the search for Brandon continues.

"I was told, 'You know ma'am, he's an adult, he has a right to be missing,'" his mother, Annette Swanson, told the Star Tribune in 2009. Wandtke didn't own a cellphone because he didn't believe in them, police said. His bank account and credit card haven't been used.

An examination of his car didn't yield evidence of foul play, although official results are pending. If he's like 95 to 97 percent of St. Paul's 1,100 missing persons cases from 2011, he'll eventually show up unharmed. But maybe not -- which is why missing persons advocates stress the importance of immediate action.

Data from the FBI show 42,678 active cases involving people age 19 and over, just slightly more than that for people 18 and under (42,480). The highest volume of adult cases occurred in the 50- to 59-year-old bracket -- 7,486.

"People over 18 go missing as well, and they deserve protections," said David Francis of Stillwater, whose 19-year-old son went missing in 2006 while exploring Idaho's Sawtooth Mountains. "We can always do better, but in Minnesota we're making strides."

Sgt. Marty Earley of the Bloomington Police Department said Brandon's Law helped find a Bloomington man who went missing in New York, where he worked during the week. Local authorities entered his information into the database of the National Crime Information Center after New York authorities declined to get involved. He was identified a few months later when his data matched remains found near train tracks in upstate New York, where he had apparently committed suicide.

"Had we not done all of that, he would've been buried as a John Doe in New York because no one knew who he was," Earley said. "Whether they chose to go missing or not, there are people who care."

Authorities were slow to investigate Jon Francis' disappearance because of his age. His family took over the search themselves and Jon's remains were recovered a year later.

"Our innocence was destroyed," said his father, who founded the Jon Francis Foundation to advocate for missing adults. "We just assumed that if our loved one went missing ... that law enforcement was motivated, trained and funded to find the missing."

Francis said Brandon's Law is a victory, but there are still gray legal areas.

Todd Tweedy, 47, of St. Paul posted a cryptic message on Facebook in early January that touched on his battle with depression. Then he disappeared. He was found unresponsive a few days later in a motel in Baldwin, Wis., and was taken to Regions Hospital.

Unlike Tweedy, Wandtke is not considered a risk to himself. If authorities find a missing person who is mentally sound, Paulos said, police can't necessarily take him to a hospital or back home or arrest him.

Missing persons expert Jeff Hasse said cases such as Wandtke's -- leaving in a car in an urban area -- can be challenging. "Right there that limits our effectiveness," Hasse said.

Wandtke's family and friends now are searching themselves, having scoured the banks of the Mississippi River in early March. The results have been disappointing.

"We can't really believe it. It's just frustrating and sad," Pepin said.

Law enforcement, no matter how well-intentioned, often does not have the resources to search regularly as days stretch into weeks, said Francis, who is helping search for an Arkansas man missing in the Utah desert.

"We have no thoughts anymore," Pepin said. "We don't know. We just want answers."

Chao Xiong • 612-270-4708 Twitter: @ChaoStrib