Quote
When a player has available to him unauthorized information from his partner, such as from a remark, question, explanation, gesture, mannerism, undue emphasis, inflection, haste or hesitation, an unexpected* alert or failure to alert, he must carefully avoid taking any advantage from that unauthorized information.
(The emphasis is mine).
A player who has not heard what his partner said may still have UI "available to him" if he was in a position where he
could have heard it.
The laws do not expect TDs to determine whether the player did hear what his partner said, only that he could have.
Law16B1{a} said:
After a player makes available to his partner extraneous information that may suggest a call or play, as for example by a remark, a question, a reply to a question, an unexpected* alert or failure to alert, or by unmistakable hesitation, unwonted speed, special emphasis, tone, gesture, movement or mannerism, the partner may not choose from among logical alternatives one that could demonstrably have been suggested over another by the extraneous information.
Law 16B3 said:
When a player has substantial reason to believe that an opponent who had a logical alternative has chosen an action that could have been suggested by such information, he should summon the director when play ends. The director shall assign an adjusted score (see Law 12c) if he considers that an infraction of law has resulted in an advantage for the offender.
If the TD determines that a player could have received UI, and that he took an action that demonstrably could have been suggested over an LA by that UI, then assuming there was damage to the NOS, he will, as law 16B3 requires him to do, adjust the score. This is so even if the player concerned adamantly maintains that he didn't receive a thing.