Dear Sam,
You could have written about the incident here. If you had written the draft of the letter, we could have corrected it. There is nothing like a disciplinary letter. Either it is a warning letter or a show-cause notice. Make it a point to include the following:
a) What happened? Has it happened before, and was any verbal or other warning given?
b) When did it happen?
c) Where did it happen?
d) Was there any loss to the company's property?
e) Who else was involved?
f) Lastly, enumerate the consequences if such acts are repeated.
Ok...
Dinesh V Divekar