Rows NOT found in cursor search

Good day everyone.

I need to update a table whose key is a customer number, say a NUMERIC(9,0).
I have to search with a cursor, for example like this:

EXEC SQL DECLARE CSR0001 CURSOR FOR
SELECT * FROM CUSTOMER_MASTER WHERE CUSTOMER_NUMBER IN (
123456789,
234567890,
345678901,....)

and assume that it is a literal very large list of customer numbers, say
over 20,000.

I need to do a simple update of a column for some status for each found
row. But I need also to report which row was NOT found in this search in
order to do some other task.

I suspect I should use a MERGE for this, but if I only use this cursor
approach, how does one know which rows are NOT found within the literal
list while the engine is searching?

Is there a kind of "exit" event to capture when the engine DOES NOT find a
matching row?

Thanks in advance.

JS