from www.tmz.com – Here’s Nick Stahl’s 2012 in a nutshell — missing, found, missing, found, and now … arrested for “touching himself” in a porno store, TMZ has learned.

Law enforcement tells us, LAPD undercover vice officers were conducting a routine check of an adult store in Hollywood around 6PM — and found Stahl alone in a private booth, watching a porno, and committing a “lewd act.”

We’re told the “Terminator 3” star appeared to be “touching himself” — you can figure out the rest — and he was booked for lewd conduct … a misdemeanor.

Nick was cited and released from police custody a few hours later … and as he walked out of the police station he told our camera guy it was all a “misunderstanding.”

As we’ve reported, Stahl has disappeared — sometimes for several weeks — multiple times this year. As of July, he had reunited with his wife and was getting treatment for substance abuse.

Kinda fun fact: We’re told one of the officers who arrested Stahl, also busted Fred Willard in July for pulling a similar deal in an adult theater.