My wife spent a lifetime on the back stretch of tracks , so I asked her if grooms piss in the horses stall, she said all the time, the horses do it.
If they are testing at those levels, picograms, if the horse would eat the straw that the groom just pissed on, logic would have it that the horse would ingest whatever toxins are in the urine from the groom. The problem I have with trainers , BB included, is not running a tighter ship, they should tell the grooms not to piss in the stalls, how about wearing latex gloves so as to not cross contaminate with lidocaine, BB said he needed to run it clean like a hospital, not a barn, and he did not.