Need help defining tests for finding Foreign Material in Product

Good Day everyone, 

I recently had a customer call in regards to finding dirt particles at the bottom of their plate. They used a marinate. I am trying to find the culprit. I have narrowed it down to two ground ingredients which are predominate in the product, garlic and onion. I know both are ground spice, I ran a filth test. What test can I do on the product or suspected RM?

https://www.fda.gov/...levels-handbook

Am I approaching this the wrong way? I have gone through our sanitation documents which showed we washed the machine prior to filling, followed by an atp test. 

First and formost, did you see the picture/got sent back the product from this customer that is complaining? Have you made sure it is "dirt" and not something else? 

I received and reviewed a video. 

The customer used up all of the product therefore could not send it back. I pulled a bottle from the lot and it is currently in the lab. It is just deciding what test to have the lab do.