what is TATP made of that he could have stepped in to trigger the false positive?
It is fairly easy to make from readily-obtained chemicals. There is no civilian use for TATP. One of the precursors could conceivably give a false positive, but that seems very, very unlikely.
"what is TATP made of that he could have stepped in to trigger the false positive?"
Almost all explosives are nitrates. If you've been to a farm and stepped in manure or artificial fertilizer, you'll set off any decent chemical sniffer looking for nitrates.
What are nitrates? Cheaper than day rates. :)