If you know the bank, contact the bank’s management directly by letter. Be sure to enclose documentation of your identity (e.g. copy of passport) and your entitlement (e.g. copy of inheritance certificate), along with any copies of bank statements, etc., available.
A direct inquiry has the advantage that it also covers client relationships that have not yet been reported by the bank as dormant or without contact. In addition, the bank can also provide you with information about closed accounts, provided that the account was closed within the last 10 years (legally prescribed period for storing documents).
If a bank rejects your solicitation, then assets can normally neither be located at that bank by means of a centralised search. Hence, a search only makes sense, if it cannot be ruled out that the assets sought could be deposited at another bank in Switzerland.