After the two drive away, Conan says that they won't follow again. Also he is only able to say that the car that followed them and the car that was parked outside were the same because of the cigarette smoke from the drivers side where the man was sitting. We don't actually know that the car following Yukiko's rental car had two people in it or not during the day time.
My theory is that the two people in the car are Shinichi's parents.
Yusaku had been driving the car during the day by himself to follow them. Yukiko finds a way to dismiss herself from the group after arriving by saying that if the show starts late she will miss her plane. She also tells Conan that Haibara has been looking at him a lot, meaning that she has been watching Haibara's movements closely. When Conan points out to Yukiko that the rental car had been followed, and that it is a possibility that it may be related to the Black organization, she immediately dismisses it and says that they were probably fans of hers. When Yukiko leaves from the group she can then meet up with Yusaku and hangout outside the window of the apartment that they know the Detective Boys are staying at because the owner had told Yukiko that they would be there. This also accounts for how they knew to drive past Aagasa's car while smoking to say that they can't follow anymore. Conan says that Asgasa's car is parked somewhat far away and they still knew to drive past it, if it was two still unknown Fbi agents or two Black org members, they wouldn't have known to do that. Also this means that Shinichi's parents have contact with Akai Shuuichi in some way.
Also, three full cases after this (Halloween ship part) which is probably the Conan equivalent of a week, she shows up again hiding in her own house. When Conan goes in to investigate it he says something like "This would be the perfect place to spy on Haibara and I from" and "by the movement of the (electrical) meter there would only be one or two (people in the house)", meaning that Yusaku might have been there as well. At this point she disguises Conan as Haibara to allow for the events at the end of that case to go as planed. Also the hat that Heiji uses for his invisible man disguise is very similar to the hat that the glasses woman wears above. Yukiko says that she made that disguise (and that it took 6 hours to make). Heiji also says that she had been hiding in her own house to observe Conan/Kudo, meaning that she probably didn't get on the plane that she mentioned she need to catch in the case above.
At the pier Vermouth says that she had arrived at that pier earlier disguised as Jodie and told all of the FBI agents there to leave. However if Yusaku was still in his own home to keep an eye on sleeping Haibara, he could defiantly inform Shuuichi that Haibara had gone to the pier with the tracking glasses. Which then puts Shuuichi arriving right after Haibara gets there.
Again in the recent SCARLET ARC, Conan then wonders how his mother was able to disguise him so well as Haibara back then after only meeting her for a short time, they also mention that she is really good at making disguises of people she knows. If she and Yusaku had been trailing her before that, they would have the information to be able to put that disguise together.
(I hope all of the images post correctly)
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If all this does fit together, does this mean that its possible for Yukiko to be sister to Shuu's mother? This would explain how they know each other... and if thats true maybe Shinichi met Sera Masumi before because they were cousins, but then Sera had to go back home and only just returned to Japan after years apart.
