I would guess milk, if it in the stomach. Any age other than adult should be expected to have some milk in the stomach. There wouldn't be pus inside the stomach.
If there's a large amount of white liquid inside the abdominal cavity, that's possibly worth troubleshooting. Organ failure and other conditions can cause an excess of peritoneal fluid, and if it white I suppose that could be a sign of bacterial peritonitis. I'm not sure how common that would be in feeder rodents.
Are there any other signs that this might be a less than high quality supplier?
I'm a little curious how you notice that there's anything in there, as usually (at least for snakes) the prey is fed whole.
I feed the rats whole to my BP, But with my cats it gets a little messier.
I think its only the weanling rabbits. Thank you guys!!