Wind slab and the seam you cannot see
Wind does not just move snow; it processes it. Grains picked up and bounced along the surface break into smaller fragments, and those fragments pack far more densely when they redeposit. The result is a slab: a cohesive layer sitting on whatever was there before.
The problem is what is underneath. If the previous surface was faceted crystals, surface hoar, or anything else weak, the slab now rests on a poor bond over a wide area. Slabs distribute load, which means a disturbance at one point can propagate along the weak layer well beyond where the weight was applied.
Two field observations help. First, slab sounds hollow, a drum-like note rather than the dull tone of unconsolidated snow. Second, slab forms on lee slopes, so knowing the wind history of the past several days tells you roughly where to expect it, and cross-loading means gully sides count as lee even when the ridge does not.
None of this substitutes for a pit and a proper stability test. It does tell you where to dig.