Heres a system that MIGHT be using piezos to dedect BB sounds
http://gprsystem.co.kr/
But no information given at all, it can't be "pressure sensitive" stuff since that wouldn't be that easy to build.
M.I.L.E.S. system-ish laser tagging
http://www.lasertagparts.com/mtdesign.htm
Check out the receivers and optics, those stuff have long range and receivers pick signals really nice, also hidden in neatly build domes.