Nick Stahl Arrested -- 'Terminator 3' Actor Busted for Going Solo in Porn Store

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" (see above). Also worth noting ... no one picked Stahl up from the station ... he just kinda wandered off into the night.

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.