//------------------------------//
// Following your breadcrumb trail to my madness 
// Story: Book 1 - The Behemoth came to Canterlot 
// by Equimorto 
//------------------------------//

It was a short bridge, over a river not too wide, near the middle of town. It wasn't an old or historically significant bridge, though it wasn't particularly new either. It wasn't really a bridge anyone had much reason to care about, beyond caring for the fact that it was there and they could walk over it. So when the bridge stopped being there, it wasn't met with any kind of particular despair, but it still provoked all those feelings of annoyance and worry that a bridge collapsing would reasonably cause.
It happened in the middle of the night, while no one was there. No one got hurt from it. But a bridge falling was something that needed an explanation, especially if it had done so on its own, and so research was done to figure out what had happened. It took a bit, not too long but at least a couple days, to make sure all the pieces were recovered. Some had fallen in the river and needed to be pulled out. At least they'd been too heavy to get carried away by the lazy current.
At the end of it, the ponies in charge of checking concluded that there had been something wrong with the support beams underneath the bridge. The something wrong being that they lacked a structurally integral section. Not in the sense that they had been built without one, but in the sense that that portion had no longer been there when the bridge had collapsed. More than one beam had been reduced to a set of two opposing ends, without a middle to connect them. Not broken, not torn off, it looked like an impossibly clean cut and the middles just weren't there. Nor anywhere else, for that matter, no matter how hard they were searched for.
For a while, ponies just accepted it. After all some reason had to exist for what had happened, even if they couldn't figure out what it was. Spontaneously disappearing chunks of matter was not exactly a structural flaw or natural event that could be planned against, so the bridge was rebuilt and that was that. The most accepted theory was that it had probably been a unicorn who'd for some reason stolen the beams. No one had any idea why they might have done that, but it was the most reasonable explanation.
And the citizens would have probably forgotten all about it, given enough time, had it not been for what happened to the town hall a week and a half later. Again, it happened during the night, and only at dawn did the ponies notice the results. There were holes in the building's front wall, rectangular, the edges clean. Sections missing from the sides of the decorative columns flanking the entrance. One hole in the roof, too. It was a work too complex and precise to have been done with anything other than magic, in a single night and silently.
Unlike with the bridge, though, the damage done to the town hall came with a signed note.