Dear Jimmy,
The question that comes to mind is that while leaving the company did this employee take proper clearance? If yes then why this recovery process was not initiated that time itself or is it that he abandoned his/her employment? If latter is the case then only you can send a letter to his home address. If it is former then you should take disciplinary action against the employee who allowed this person to go.
While I am not saying that you should not send the letter to this person. That person should have returned the company items whether anyone asks or not. However, draft of the letter would depend on two factors i.e. (a) the employee abandoned the employment (b) separation of the employee was as per normal procedure however, recovery of the company items was escaped from the attention while clearing him.
Thanks,
Dinesh V Divekar