Here’s the thing: has the DPRK army moved a large number of troops close to the DMZ in recent days? Or has there been large movements of trucks loaded with ammunition to artillery positions north of the DMZ that can reach Seoul at the same time? Any invasion will require a large massing of troops, and that’s really hard to miss with modern sensor technology.
Lots of underground tunnel complexes in that area so the NORKS can move that stuff underground...