Sudden gap between disc brake and pads?
It sounds like what probably happened is you did the kind of ride where the brakes are exposed to enough wet grittiness that significant pad material is lost over the course of one ride. This is also kind of the hallmark disadvantage of organic pads, if that's what you have. The 3mm pad adjusters primarily exist to compensate for pad wear, so that's what you should be doing to narrow the gap again. The idea is that eventually when the pads are all the way worn, you can then screw them back out and not have to adjust anything else for the new pads.

Another element to this can be that any initial error in parrallelism between the pads and the rotor will also result in the gap widening quickly as they break in and wear into a more parallel state, which you would also use the adjusters to correct. Some amount of this is inevitable although you want to avoid it as much as possible with good initial adjustment and prep. It's unlikely you experienced much of this here unless those first 1500km were somehow in very mild conditions without much need for the brakes.