Thanks 737aviator. They do use some anti ice fluid here in Canada now to try to help manage runway contamination (if you want to nerd out on ice control chemicals there's an AC on it below) so maybe it could be injested too, but you got me thinking about how much residual Type IV there is on a day like that sitting on the runway after having sheared off the previously departed aircraft. In the daytime you can sometimes see a green slick part way down the runway where most aircraft reach the shear speed for the fluid. So perhaps that could be kicked up as well.
Either way, now that I have been forced to consider the APU inlet's position it's no wonder fluid finds its way in there. I imagine it probably find its way there from any of the avenues mentioned: poor application by the deice truck (unlikely in this case), fluid streaming aft on takeoff after shearing off the wings and perhaps even getting kicked up off the runway onto the fuselage where it could stream back (I don't think it should be able to spray directly into the intake as it has a deflector but I don't think this would protect from fluid flowing along the skin).
Anyway, I for one will not be taking off on the APU bleeds again if there is any deice fluid anywhere.
Cheers.
TC Ice Control AC:
https://www.tc.gc.ca/eng/civilaviati...-300-1762.html