How do you prove a computer is yours and is this an illegal search?

I heard an interesting story from a friend the other day– he was biking in to work with his laptop bag on his back and a cop pulled him over. He wasn’t sure why he wasn’t being pulled over (maybe it was the bagel he was eating while biking), but he stopped and the officer started an interesting conversation. He asked my friend if there was a laptop in his bag, which seemed pretty likely considering it was a standard laptop bag. My friend answered, yes, at which point the officer pointed out that there had been some laptop thefts in the area. He then asked my friend if the laptop was his and if he could prove it.

Despite the strange request, my friend just wanted to get on his way, so willingly cooperated with the officer and was able to show the officer the address book on his computer which included his own information that presumably matched his ID. This was apparently enough to satisfy the officer and he sent my friend on his way.

Now, this situation brings up a lot of interesting questions– I’m no lawyer or legal expert in anyway, but it seems like to me that the officer would have to have some type of probable cause to stop my friend and not only search his bag, but in an effort to “prove” ownership of the laptop, search the contents/data of the laptop itself. Young people biking with laptop bags isn’t a rare sight in Palo Alto. Did my friend match the description of potential suspects? When he saw the laptop, did it match the description of recently stolen ones? Presumably, if the officer was aware of laptop thefts in the area, police reports have been filed and serial numbers should be available for those stolen computers. Finding the serial number on a laptop is relatively easy– if the officer did have probable cause to believe this laptop was stolen, he could have easily run the serial number.

But aside from all that, how did the officer expect to have my friend “prove” that the computer was his? Out there on the street, without purchase records, how do you prove that a computer is yours? Maybe you use your real name as your login name, but if you’re unlucky enough not to, you might have to show your address book or other private information to the officer in an effort to show that you’re not a thief. What is sufficient evidence in this impromptu courtroom out on the street? And what if the laptop isn’t yours? That doesn’t mean it’s stolen– plenty of students in my office borrow laptops while working for us and have generic logins and don’t necessarily keep any personal information on them. Then where are they left?