I spent a few days with the Brandenburg State bomb squad in the late 90s. They find a lot of these. We didn't declassify our post-action photos until the 1950s, when many German cities had been rebuilt, and the bomb disposal teams scour those photos for evidence of small holes where "duds" landed. A lot of time, they're under buildings, and are just marked for removal when the building is demolished. If the bomb is deep enough, an explosion will probably not bring the building down, but crack it and force its evacuation. For those bombs they do identify in the open, they use metal detection or ground penetrating radar to confirm the bomb's location, they set up a priority list and then have "bomb days." In Oranienburg, where I was with them, they would put it in the newspaper, and everyone would go shopping in Berlin that day while they dug up several bombs simultaneously and either trucked them out or blew them in place.
A lot of the bombs had a fuse involving a ceramic cap that was broken on impact, allowing an acidic chemical to seep into a charge and set it off. This was a way to create a time delay, otherwise, your bombs would blow up at the top of a building and only damage one floor. The Germans (and Brits, since the Germans used something similar) accused their enemies of making bombs targeting first responders, but it was really just time delayed fuses that went off later than they were supposed to. The devices were frequently imprecise, and might not impact (and therefore set off) as intended every time. These bombs were designed to be set off by extreme impacts, meaning buildings, but a lot of time, if they hit dirt they would simply burrow in the ground.
Bombs in rural areas are poorly mapped, because there's generally not post-strike imagery of something that missed or wasn't even near its target. So something like this wouldn't be on anybody's list. It could have exploded by the cap finally giving way, or by the acid working its way around the corroding structure. In this case, it was probably deep enough to not be struck by a farmers' implements (when I drove a tractor growing up, nothing I pulled went more than 2 feet under ground). It probably just corroded and failed and blew up on its own.
