What just happened?

I am walking through the train station today on the way to the gym and a Japanese guy stops me and starts asking me questions I can't understand. Then of course I forgot my phrase to tell him I only spoke a little, so I just stood there. He then pulled out a police badge and some sort of ID. He was wearing street clothes and about 25, maybe 30? Who knows. My first thought, is this a joke? He is trying to scam me? Is he really a police officer? (I chalk my paranoia up to watching too much TV and reading too many warnings on the Internet about people posing as police officers. But come on! We are in Japan.)

Then he says passoporto something...
I said, "I don't have my passport. "
He says, "Sorry, passoporto carry/require" ( not really sure )
I said, "I am an American?"
He says, sorry, passoporto thing....again
I said, "I have my alien registration card?"
He just looks at me and apparently he isn't leaving until I produce something for some reason I have no clue. I look around, I don't see any officers. I pull out my alien registration card and show it to him.
He says, "Oh, sorry, sorry!" All the while smiling and giving me a little bow.
I said/motioned, "Can I go now?" I was clearly annoyed.

And apparently I was free. What just happened? Seriously, did he think I was a criminal in my gym clothes, spring coat and YMCA bag on my shoulder?