It is difficult to say with precision what coins are at risk. The general answer is that the coins that are found buried or have been in contact with the soil for a period of time. It is not HCl that is the direct cause, as copper does not easily react with the acid. Here is a bit from my latest paper, as yet in finished.
What Happens and To An Extent Why.
Copper in moist air slowly acquires a dull green coating, called Verdigris, because its top layer has oxidizes with the air. Some architects use this material on rooftops for this interesting color. The simple chemistry is as follows:
The green material is a 1:1 mole mixture of Cu(OH)2 and CuCO3.
2 Cu(solid) + H2O + CO2 + O2 → Cu(OH)2 + CuCO3(s)
Malachite is (Cu2(OH)2CO3). Copper II can be generated by treatment of the hydroxide, oxide, or copper(II) carbonate with hydrochloric acid. In many cases it is the simple formation of Hydrochloric acid from Sodium chloride [NaCl] and water [H2O].
NaCl + H2O → NaOH + HCl
2NaOH + 2HCl + CuCO3(s) = NaCO3 + CuCl2 + 2H2O
Why is it so hard to detect?
Anhydrous Copper II Chloride may appear in color from a dark brown to the cyan green, depend on the amount of atmospheric moisture that is absorbed. If it was in a desert region with very low relative humidity and dry air, it may never be seen. In moist or very humid areas, it changes color rapidly and starts to work on the solid copper. See the picture I posted earlier. The color change of the copper II does explain why Bronze Disease is so hard to detect.
Back to your original question. All very old coins, especially finds by metal detectors, even small silver coins, should be treated in a soda bath. this should be done as soon as possible after their exposure to air. I don't mean radical attacks on the coins, but get the mud, clay and dirt off, and allow the coin to soak in the soda bath or a high pH bath for a day or two. This may not eliminate the problem, but will help minimize the damage.
Exposure of any copper based alloy to salt spray or environs that have common salt, I am referring to NaCl or basic table salt, in the air, may be at risk, but it is limited. Think about modern coins, they are just as exposed, but because of being rubbed and handled , they for less of the needed oxides. The oxides are part of the equation.
I hope this helps.
Bruce