I don't think that's enough for LSD. Minimum effective dose for LSD is on the order of single digit micrograms per liter of body volume. A picogram is a trillionth of a gram - that's a million times smaller.
I bet it's some kind of noise. To prove a detection at the picogram level requires a herculean effort to prevent contamination. A positive test at the parts per trillion level has to be repeated under stringent protocol and confirmed by a separate means before conclusion. Protocol alone doesn't prove the conclusion.
How do you determine the body volume of a horse? The only way I can think of is to immerse the horse in water and see how much water is displaced. There would be a significant difference between the horse in an exhaled state vs. the horse in an inhaled state.
A more likely measurement is picograms per liter of blood or picograms per kilogram (mass of the horse).