Search service lost SQL permission

Summary

While working with Result Sources in a site collection, I got that wonderful "Something went wrong" error. The ULS showed a bunch of SQL execute permission errors on the search database.  The solution was to update the SQL permissions for the search service.  It was not obvious why the permissions where lost in the first place.

Symptoms

ULS Error events such as:

System.Data.SqlClient.SqlException (0x80131904): The EXECUTE permission was denied on the object 'proc_MSS_GetLease', database 'SP2013_DEV__SA_Search'

…and other SQL permission errors.

Fix

Grant SPSearchDBAdmin role to the search service account on the Search database.  (SQL Server Permissions)

Thoughts

Just before this occurred I had updated permissions of the Search Service Application in Central Admin.  I had added my user account.  Can't see how this would have caused it however.