I think I have found the source.
It being several users accessing the site via UK 3G/GPRS providers who modify the content.
https://kc.mcafee.com/corporate/index?page=content&id=KB52171
####Problem
SaaS Endpoint Protection fails to update and you see the following error message in the update window:
BMI_LOAD is undefined
The SaaS Endpoint Protection system tray icon eventually shows the error condition symbol.
####Cause
BMI_Load is a JavaScript used by various mobile vendors in General Packet Radio Service (GPRS) modem software. In other cases it is injected into HTML by mobile data carriers. This JavaScript reduces image quality to improve bandwidth over the GPRS connection. The BMI_Load JavaScript can conflict with SaaS’s JavaScript resulting in an error condition that prevents SaaS Endpoint Protection from completing the DAT update.
Possible solution
Consider adding Cache-Control "no-transform" header.
However this might have other ramifications in terms of intermediate caching servers, but as long as this isn’t included on the image files it shouldn’t have that much of an effect.
EDIT: If a site is behind Google PageSpeed - this header would cause the page not to be rewritten, perhaps impacting CloudFlare sites too… personally I would prefer knowing the site is loaded without errors.