The Elder Scrolls XVI: Sworn to Carry Your Burdens

[Silverraptor]

Okay. So there might be something that allows dragons to sense other dragons. Still, Alduin's initial reason to go to Helgen was probably to free Ulfric. After all, you were just a nobody who hoped the border and got thrown in with the other rebels. When Alduin appeared, he may or may not have sense you. Maybe not because you hadn't absorbed a single dragon soul so maybe it was unnoticeable. On the chance that he did sense your soul among the crowd, that might have been that he reasoned that since Ulfric knew the ways of the Voice, it might be messing with his dragon souls sense and thought sensing you, that you were Ulfric. Maybe even intentionally avoiding killing you thinking you were Ulfric. But that is conjecture.

I still think Alduin went to Helgen because he wanted to free Ulfric for his own purposes. Whether or not he realized a Dragonborn was around when he got there can be up for debate, but it wasn't the initial cause for him going there.