Pocket protectors? Slide rules? Any other items nerd/engineers carry in their shirt pocket that falls out when they are head-first into the raised floor.
We've found socks and cigarette butts. Needless to say the perp is no longer with us :)
Like most disasters, you may not be able to prevent flooding, but you can mitigate some of its affects. Hit the E.P.O. and unfurl the plastic tarps you have stored & handy for just this emergency.
We also put drip pans which cover the entire ceiling over the data center - we were burnt 2 years ago in a pre-war office building in NYC. (avoid space with wc's above!) The pans drain outside of the machine room into a high-traffic hallway (with a drain). Any drips get noticed very quickly, and we don't have to test the water alarm.