Cat = catalytic convertor.
Spilled milk/baby's bottle?Can be very pungent!! It could be the heater matrix leaking into the heating system although that smells more like urine.Are you losing any coolant?
Don't worry, I do know that I was referring to where you said "once the cat is run in"
In fact, my preference of smells would likely be catalytic converter, puke then cat. Cats of the animal variety are not my thing!