I have an excellent sourcewho has assured me that cocoa powder is in fact added to the Lindt Excellence bars at the end of the refining period, to increase the intensity of the dark chocolate flavour.
With regard to your question about whether or not they need to disclose this, the answer according to the FDA is "No".
Here is an exerpt directly from the FDA's website:
Sec. 163.111 Chocolate liquor.
(a)Description. (1) Chocolate liquor is the solid or semiplastic food prepared by finely grinding cacao nibs. The fat content of the food may be adjusted by adding one or more of the optional ingredients specified in paragraph (b)(1) of this section to the cacao nibs. Chocolate liquor contains not less than 50 percent nor more than 60 percent by weight of cacao fat as determined by the method prescribed in 163.5(b).
(2) Optional alkali ingredients specified in paragraph (b)(2) of this section may be used as such in the preparation of chocolate liquor under the conditions and limitations specified in 163.110(b)(1).
(3) Optional neutralizing agents specified in paragraph (b)(3) of this section may be used as such in the preparation of the chocolate liquor under the conditions and limitations specified in 163.110(b)(2).
(4) Chocolate liquor may be spiced, flavored, or seasoned with one or more of the ingredients listed in paragraphs (b)(4), (b)(5), and (b)(6) of this section.
(b)Optional ingredients. The following safe and suitable ingredients may be used:
(1) Cacao fat and cocoas (breakfast cocoa, cocoa, or lowfat cocoa);
(2) Alkali ingredients. Ammonium, potassium, or sodium bicarbonate, carbonate, or hydroxide, or magnesium carbonate or oxide, added as such, or in aqueous solution;
(3) Neutralizing agents. Phosphoric acid, citric acid, andL -tartaric acid, added as such, or in aqueous solution;
(4) Spices, natural and artificial flavorings, ground whole nut meats, ground coffee, dried malted cereal extract, and other seasonings that do not either singly or in combination impart a flavor that imitates the flavor of chocolate, milk, or butter;
(5) Butter or milkfat; or
(6) Salt.
Breakfast Cocoa is defined as:
Sec. 163.112 Breakfast cocoa.
(a)Description. (1) Breakfast cocoa is the food prepared by pulverizing the material remaining after part of the cacao fat has been removed from ground cacao nibs. Breakfast cocoa contains not less than 22 percent by weight of cacao fat as determined by the method prescribed in 163.5(b).
There you have it. Any manufacturer can add cocoa powder to the cocoa mass, and simply call the lot of it "cocoa mass", or in Lindt's case "chocolate". Kind of deceiving, isn't it?