Flashback to Doncaster station…
Remus and Sirius had paid a visit to Tonk's grandmother Elaine's house situated in a leafy muggle suburb on the outskirts of Doncaster. The reason being for the visit was Elaine really wanted to meet Sirius but never had the chance to, especially given the 12 years in Azkaban Sirius was forced to serve. Upon hearing he had escaped however, Elaine demanded that Sirius get himself up to her house right away.
Of course, given that Sirius was still a wanted man, there was no way Remus was going to allow Sirius to go galavanting off on his own, he didn't quite feel like he could trust Sirius to behave himself on his own and not get exposed and caught. Yes, he had the option of his animagus disguise, but what if he got captured as a dangerous dog instead of a human? Remus would never be able to live down the pain of losing his only surviving friend a second time either through being returned to Azkaban or killed as a dangerous dog. So he practically held Sirius at wand point until he agreed to let Remus accompany him.
Although both Remus and Sirius were easily able to apparate to and from places, Sirius had said he'd always wanted to travel on a muggle's long distance train service at some point in his life, much to Remus's amusement. Seeing as they weren't in a particular rush, he agreed however. Remus even enjoyed it, even if he did get funny looks from many fellow passengers for taking with him Sirius in his large black dog form. Fortunately though, Sirius behaved perfectly well the entire journey.
The visit, to Sirius at least felt like a complete waste of time, since much of the time spent talking to Elaine was spent on lecturing him on 'behavioral modification'. With Remus however, the conversation between himself and Elaine were seemingly far more fulfilling.
Once they made their way back to the train station, Lupin grabbed himself a quick coffee from a nearby coffee shop and sat down with the dog form Sirius at his legs. A train arrives from London, no good to them, they need a train back to London, but it was Sirius's reaction that had Remus utterly perplexed. Sirius started sniffing l, then barking very loudly and Remus had to restrain Sirius from going anywhere near the train. Once the train had departed, Lupin gave Sirius a very stern telling off.
A few hours later when they returned to Grimmauld Place, however Sirius gave an explanation. "Remus, I swear on Merlin's life I could sense and smell Harry! But I couldn't see him anywhere, not on that train or at anywhere in the station." Looking aghast, Lupin replied "But that's nigh on impossible, Harry would have been on the Hogwarts Express, not on a random muggle train! Sirius, I think you miss him so much, and hanker for all of those lost days you should have been spending with him instead of being in Azkaban, your brain is addled." "Nonsense!" Sirius snapped, "I recognize my Harry when he's around, invisible or not. I gaurentee you his was near us in some way or another!"
Before Remus could retort, the fireplace burst into green flames and out emerged none other than Albus Dumbledore. "Lower your wands, you too, it is only me, Albus Dumbledore", he replied, slightly amused by the reaction of the two with their wands out. "Well, maybe if you had given us some warning first before the random intrusion into my house….." retorted Sirius with a sneer worthy of Snape, but Lupin put a hand on his shoulder to silence him.
"Sorry about the sudden intrusion, I appreciate the fact that it is indeed your house, but it is emergency, so I am calling an emergency Order meeting meaning you can expect the 'intrusion' of several other of our Order members very shortly" Dumbledore now looked very serious. "Oh, how so?" asked Remus.
Before Dumbledore could explain, the fire place burst into life again and much to Sirius's displeasure, Severus Snape emerged from the emerald flames. Both Sirius and Snape exchanged looks at each other like they had seen an unwanted rodent.
"What could possibly be so important you had to disrupt my latest potion making project, forcing me to have to restart it later at my own great expense?!" he hissed.
Again, before Dumbledore had the chance to reply, Minerva McGonagall emerged from the fire place, followed by Mr and Mrs Weasley, then Bill and Charlie Weasley, then Kingsley Shacklebolt, Tonks, (much to Lupin's immense pleasure and she was sporting long, shoulder length white blonde hair) and Mad Eye Moody.
Dumbledore motioned everyone to sit around the kitchen table. "Are you finally going to explain the sudden 'house party'" Sirius demanded. "All in good time my boy" Dumbledore admonished. He then conjured up for himself a bottle of tonic.
"I have called you all here at such short notice, which I apologize for due to the fact that it has come to my attention Harry Potter is missing". Everyone else sat around the table looked like they had just seen Voldemort appear.
Something clicked in Remus's and Sirius's minds causing them to look at each other in horror, hadn't Sirius's claim of sensing Harry at the train station in Doncaster in fact not merely being a false alarm brought on by Sirius's fragile mental health and they had missed out on a crucial opportunity to intercept Harry? Do they mention it to Dumbledore or try and get to the bottom of why Harry was way off course and what ulterior motives he had for doing so?
Their horrified looks hadn't gone unnoticed by Dumbledore. With a brilliant twinkle in his blue eyes and cocking his head slightly, "Is there something you two gentlemen wish to share with me?"