What happens if the testator / beneficiary goes missing?
In terms of Estate Planning, until the person is legally presumed dead, you cannot act accordingly as if the person is dead.
You should file a missing person's report with the police once you have reasonable grounds to believe a person has gone missing. Ultimately, it has to be approved by the Singapore Courts before a person can be legally presumed dead. There are also many factors to consider before the missing person can be legally presumed dead, the arguable more popularly known is that the person is missing for 7 years, though it is not always ironclad proof.
Source: Evidence Act 1893, Singapore Statutes Online website
This means that for the missing Testator, his/her loved ones cannot proceed with the probate/administration process which requires the death certificate of the testator.
Likewise, for the missing beneficiary, the executor should not redistribute the beneficiary's share of the inheritance (depending on the clauses in the Will) until he/she is legally presumed dead.
This is similar in terms of for certain insurance products, where you will not be able to claim for the insured's death payout until he is legally presumed dead (i.e. a death certificate is issued) nor would you be able to "write-off" a missing insurance nominee's share of the payout.