You said you washed the car in the shade, however was the car warm?

When you wash a car when its still hot, if you look closely you can often see a spotty residue / water marks. You dont really notice it untill you start drying though. Perhaps it was that?

Did the residue form towards the "end" of the car (ie you started wiping it down near the boot and the crap was on the bonnet)?

Megs is an exceptional BFYB product and even their pink "soft wash Gel" shouldnt leave any residue.