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 disciplinary letter. Either it is warning letter or show cause notice. Make it a point to include the following:
a) What happened? Has it been happened earlier and any warning verbal or otherwise was given?
b) When it happened?
c) Where it happened?
d) Was there any loss to the company property?
e) Who else was involved?
f) Lastly enumerate the consequences if such acts are repeated.
Ok...
Dinesh V Divekar