There could be some strange combination of halogens / chlorides ( in the sea-water, towns water, your detergents) and / or irons or other elements in the towns water in action.
I would think that you would have to be near the sea-front for the location to have a significant effect.
If the cost / effort is not too high, you could just introduce a periodic inspection and replace them or try increasing the flushing rate with clean water. (But see my last comment)
It is easy to think of 'stainless steel' as being very corrosion resistant, but many engineers and metallurgists have found, to their cost, that given the right (i.e. wrong) conditions and presence / abscence of certain chemicals / elements, it is anything but.
Abrasion , galvanic action / attack from dissimilar metals may start the process. Once the 'passive' surface is penetrated or removed and for some reason cannot be replaced by the action of oxygen, then corrosion, from a number of sources, can occur.
Iron typically leaves a reddish-brown residue.
Rate of attack is affected by concentration, temperature, velocity of flow and stress in the metal parts subject to attack.
Typically, elevated temperatures can increase the rate of attack. Problems can occur in, or adjacent to, welded areas.
Is there a difference? i.e is the problem on the surface most exposed to atmospheric / splash action or the side most exposed to the wash process?
Do you use micro-organism based cleaning fluids / processes?
Do you do something different in the plants with the problems.
If the material is OK in machines in other areas, and the materials / mechanical process are the same, then you need to start examining specific local conditions. Start with local water analysis.
