I agree with B.K. Bhatia's comments; and the 3 questions that he has put forward.
Other relevant questions can be :
To which function, does this senior employee belong ??
What is the opinion of his Reporting Officer ??
Simply saying, "Have noticed a drop in quality of a senior employee."; is not sufficient.
"Drop in quality" of what ??
Is it dress, attitude, informal or formal participation; punctuality, relationship and interaction with other employees ??
If it is Performance, then what are the criterion, what are his tasks and targets; what is the end-date of this year's performance appraisal period and what are his ratings ?? what were the comments of his Reporting and Reviewing Officers ??
One should remember that HR is in no position to JUDGE the PERFORMANCE of employees in other functions like Production, Sales, Client Service, Finance & Accounts etc.
Neither it is competent, nor it has the opportunity to do so, as HR is not a party to all the activities of an organization; nor it is empowered to do so.
Moreover, one should not EQUATE performance with outward appearance or behaviour of the person. Condemning a person like this is unfair and injustice to that person.
Several times, in their over-enthusiasm and over-zealousness, young HR persons make the mistake of having the impression that " they are the lord of whomsoever they survey" !!!
Such erroneous thinking is bad for the organization. It is like the story of the proverbial monkey with the razor trying to become the barber - it is always possible that the neck gets slashed instead of the beard.
As far as judgement on performance is concerned; suo moto it should be restricted to one's own subordinates or those who report directly.
Hope I have not hurt the feelings of anyone, by coming down heavily on such impropriety or proposed action which may be unfair to someone for no fault of his on the performance front, which can be judged only by his KRAs under a good PMS; or to the un-biased opinion of his reporting officers.
Warm regards.