I know there are other blood and body fluid detection products other than Luminol *that is used with black light so that wouldn't be used here being out in the daylight*...Blue Star is another that is quite similar to Luminol...
I know there is iodine spray & ninhydrin...among others...
Could you ask your son about sprays in particular? I don't know if this would be powder....in the picture there are definite remnants of a spray of some kind...
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It would be awesome if we could find out what the spray product might be...thanks newstalk!