Let me share some information on how to look for these inactive customers, @TH_PERS.

An inactive customer is still searchable from the Customers page. Just make sure to include all the inactive from that page. Once done, you can click on the search box beside the Batch actions drop-down and type deleted. 

That'll show you all the customers that you've marked as inactive. Check the screenshot below:

Here's more information on how to add, delete, merge, and restore a customer. 

Please let me know how it goes or if I can be of additional assistance. I'm only a post away should you have any follow-up questions.