If the enquiry letter is sent to address given by the employee and it bounces back it is presumed that the employee is aware of the contents and is trying to avoid service of notice. The alternatives are to publish it in a local newspaper or get it pasted at the address given.
6th December 2018 From India, Mumbai
An employee has been suffering due to cancer and it appears that because of his disease he could not attend his work. Notwithstanding his absence, your company paid him the monthly wages. Your company's generousness is appreciable. Nevertheless, before disbursement of the wages, did any company visit his home or the hospital where he was admitted? When the employee started absenting, that time did your HR ask for the medical certificate? Did your HR verify the authenticity of the certificate?
When your company's administration took a decision to disburse the salary then it was their conscious decision. Now what exactly changed that merited sending notices to the absenting employee? Employee could have been kept on the rolls even without disbursing the salary. But then did your HR is smelling something fishy?
Your question is conducting the enquiry ex parte. You may do so but please check your country's labour laws. Secondly, let your HR investigate the whereabouts of the sick employee. If whereabouts are not established, then you may give notice in the newspaper and ask the employee to report for the duties. If he fails to report then you may conduct the ex parte enquiry and close the case.
6th December 2018 From India, Bangalore
The concerned employee has been suffering from cancer for last two years which is within your knowledge. You have paid his salary for one year. What charges have you framed against him for which you want to conduct ex-parte enquiry?
You asked whether ex-parte enquiry can be conducted presumably for absence from duty for one year out of two years' absence due to cancer. You know his absence is due to compelling circumstances beyond his control. What makes you think that this is fit case for disciplinary action?
If you want to ease him out, persuade him to resign by paying separation package if your management agrees.
I don't know whether your labour laws support your action given the incurable nature of illness incapacitating him to attend office which fact is known to you.
In my view he requires to be shown full sympathy and not disciplinary action.
7th December 2018 From India, Mumbai