My first run in with the Japanese Police

Yes, it finally happened....the cops are onto me, but I'm not sure why. So the other night I was biking home from work with another teacher because she was teaching with me at my school that day. I was around 7:30pm and we biked past a local police box or Koban. I went through the green light and the other teacher was behind me when the police man started running after her and throwing his arms in the air. I stopped to see what was going on and they were pointing at her bicycle seat. I was wondering if maybe her seat was too low, but I think it had something to do with the fact that she doesn't have the regular bicycle lock that is permantly attached to the back tire. i don't have that type of lock either, but I have a fold up bike and I don't think that kind of lock would fit my style of bike. Anyways, it was really confusing and finally they asked if we were travellers and they asked for our passports. Or course I didn't have my passport on me but I did have my ARC (alien registration card). That card has my address, my passort number and I think it has my company's info on it as well. After he looked at my card I was pretty sure he made a remark about how old I was and then they apologized for stopping us. So it was quite random, but I thought it was kinda funny. I'm still not sure what they were looking for or who they thought we were.