Find missing records between two tables

Do an ANTI-JOIN in Power Query.

1. Select the first table. You can create a reference to that table if you don't want to distrupt that table first.
2. Select the Merge button on the Home Ribbon
3. Configure the dialog box as below. My Database 1 Customers (2) is because I created a reference.

It returns this:

If you want to see the M code doing this, create a blank query and put this code in:

1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done

let
    Source = #"Database 1 - Customers",
    #"Merged Queries" = Table.NestedJoin(Source, {"CustomerID"}, #"database 2 - clientcustomers", {"CustomerID"}, "database 2 - clientcustomers", JoinKind.LeftAnti),
    #"Removed Columns" = Table.RemoveColumns(#"Merged Queries",{"database 2 - clientcustomers"})
in
    #"Removed Columns"