The Grasmere case - gates chain-sawed off and dumped in rivers - does look like vandalism, albeit for mysterious reasons. if the Norfolk signs were generic Public Footpath or Public Bridleway varieties, they might be re-usable elsewhere (with a saving for contractors ?), but if they're specific to that National Trail or those destinations it's hard to understand. I've seen evidence of signs tampered with to show the wrong direction, and individual signposts removed or toppled, but never multiple disappearances along a whole length of trail.
Have there been disputes with property or land owners on the this stretch (Winterton to Horsey) of the Norfolk Coast Path ? A legal path routing decision taken against someone in that area ?