First of all, well done you!
Secondly I can understand why they would ask the question; however, it does sound like another BRC auditor going out of the strict scope of the BRC standard... You may or may not be aware that the maximum residue limits on quats was reduced in the EU last year and the fear is that many manufacturers may not be compliant in the UK where we don't tend to rinse off quats.
The first thing to say with quats is there is no food safety risk. The second thing to say is rinsing them off is not required, it is an option (which at least one major retailer describes as a bad idea). I would suggest you do a risk assessment. As part of this it might be worth testing your product in worst case scenario conditions to see if quats are detected (Campden BRI do this testing). It it's not an issue, I wouldn't change anything. If it is you might decide to do one of the following:
Rinse (this might cause other issues though)
Discard the first products made from the line
Change to another cleaning product.
pH strips will obviously only work if the pH of your cleaning is different from water?