To the person who is aware they are volunteering. Anything else would undermine that persons will.
Will is something of an individual liberty especially when it comes to a persons choice to disclose their personal/protected medical information.
A clear knowledge and willingness to participate in voluntary assessment (s) would be the only way to preserve an individuals freedom to chose with consent and autonomy.
If an individuals freedom to peruse happiness, which for me personally would involve consent and autonomy, would be misinformed, compromised (or otherwise unethically ignored) the employer should be penalized.