The idea being "Here's a rough description of the guys that robbed us" and "Wait, we were robbed?" respectively.
Also, laés is an Awakened drug, so it doesn't come up on mundane scans. Should a healing mage go all wibbly-wobbly magic-diagnosis, he'll pick it up, but not the average urinalysis. Which is in and of itself not something DocWagon does when they pull up to a suspected concussion patient.