The debate concerning the effectiveness of dowsing roads is old. The oldest German book on mining from 1556 shows, among many other pictures, men with dowsing rods. The remarkably modern author wrote that while he does not think rods work some miners think it does.
Both pendulum and dowsing rods are worthless means when it comes to find anything. When the human factor is removed, i.e. when the dowsing rod is put into a vice, there is no movement of the rod no matter how much water, gold, whatever is brought in its vicinity. Also, there are no known physical effects that could create such forces. It is extremely unlikely that effects so strong have been missed by modern physicist.
The only reason why a dowsing rod moves is because the muscles of the user move. The dowsing rod just amplifies the movement. In fact, such small muzzle movements are sufficient that maybe the user is not aware of that and thinks the rod moved by itself. So not every use of dowsing rod or pendulum is a conscious attempt to cheat someone.
There have been many attempts under scientific circumstances to prove or disprove that a dowsing rod works. In all cases the "detection successes" were within the frame of what was to be expected just by chance. That means the user of a dowsing rod can as well guess. Many misconceptions concerning rods or other claimed phenomena are based on a missing understanding of the rules of statistic and physic. Just because something produced results a number of times that does not mean it is a proper technique. The number of successes must be above what is to expected by chance.
In some rural areas dowsing rods are still in use to find water. As stated above the rod merely amplifies the user's muscle contraction but for an observer it might seem that it works. Many of those who sell water detecting services are well acquainted with the local hydrogeology. Because of knowledge and experience they can judge how high the chances are to find water at a specific spot. Often these people are successful. When they carry a dowsing rod an observer might think that this success was caused by the rod which is inaccurate.
Pendulums swung over a map, a picture or whatever are similar to dowsing rods inasmuch only a very small force is needed to change its movement. In each case the human operator is responsible for the outcome, not any unknown forces between map, "hidden treasure", and pendulum.
It is interesting to observe that dowsing rods and pendulums are not used if a real detection technology exists for the desired item in question. So dowsing rods are used in an attempt to find water but not metal shallow enough to be detected by the usual technical means.
Rods and pendulums can be placed in the trash bin along with other esoteric nonsense such as "earth rays", "water rays", "force lines" etc..