Ok, I can't explain what it can be without seeing it and knowing your process but perhaps we could go through a 20 questions thing and see if we can help your investigation.
Have you shown it to the engineers and operators?
Have you tried sending it off for identification of the plastic? Campden do this service but they ain't cheap.
Do you have sieving process? Is it actually working? (Sometimes things you think are happening aren't.)
What are your audits showing in terms of foreign bodies? Do you do black bag audits? (Can you start?)
Have you contacted suppliers? Have any of them responded very, very quickly? (Sorry, I have a suspicious mind.)
I'll have a think on to see if I can think of any more lines of investigation.
So, once you've completed the investigation, if you don't find the cause (and I urge you to because if you don't this could cause a big issue. There is obviously a linked issue here which could turn into a recall depending on the politics of your company,) we could help you think of a response. I could suggest the following but again, this does sound like b******* to a customer.
Something like:
"Thank you for contacting us and sending us the piece of foreign material. We have identified it as a piece of brown plastic 2-3 mm in size comprised of xxx plastic. We do not* use this variety of plastic in our facility. On investigation we have unable to identify the source of the plastic. Our suppliers have also been unable to identify the source of the contamination. We have audited our facility and increased the frequency of audits instigating black bag audits on a daily basis etc, etc."
* obviously only write this if it's true.
Not sure if that's any good but it's just off the top of my head.
As I said, this sounds like a withdrawal waiting to happen. I'd tread carefully and find the cause if you can because having one complaint is something you could perhaps dismiss, having several complaints the same suggests you do have a problem. Even if you don't find the exact cause, you need to work on your FSQMS because something has gone wrong.
Edited by GMO, 24 March 2011 - 08:23 PM.