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You should absolutely inform HR of the additional inappropriate behavior that has made you uncomfortable. If they are not aware of it, they may just believe that he made this allegation as a means of trying to avoid the PIP and that there are no other concerns.
Your additional facts could give HR and the employer pause in allowing the employee to remain in the office, and for safety purposes, they could suspend him. Do they have to do that? No, the don't. Regrettably, the law doesn't impose a requirement on an employer to act in light of these sorts of situations. Sadly, all the law does is impose liability on an employer when they don't act and something happens. Courts count on the fact of this potential liability as the motivation for an employer to do what they should.
So, other than being able to report the behaviors to HR, there is nothing more you can do.