I am an HR manager and we have had a recent incident in the office, where a number of employees stayed after hours and drank excessively, one of them acted inappropriately, startling another employee who was still working.  I do not want to discipline these employees, but can I write a warning note about their behaviour and put it on their files to refer to, if needed in any future disciplinary proceedings?  I want them to know that the business takes this kind of behaviour seriously.  How do I handle it?

Your question is not an unusual one, given the festivities that are under way at this time of year.  However, the simple answer to your question, is no.