Hello,
Thanks for your question. The answer to this depends on the version of QuickBooks you are using with Method (i.e. QuickBooks Online and QuickBooks Desktop).
If you are using QuickBooks Online, you can find deleted customers by changing your filter to view 'All' customers. Since customer's in QuickBooks Online are made inactive instead of deleted, Method follows the same behaviour. Therefore you can make the customer active again in Method, or you can make the customer active in QuickBooks Online and it would sync vice versa. The activities, cases, etc will not be deleted from a customer made inactive (if you are using QuickBooks Online).
If you are using QuickBooks Desktop, then it's not possible to find that customer since they would be deleted in both Method & QuickBooks Desktop. This would also result in activities, cases, etc, being deleted from the customer as well.
Hope this answers your question.
best,