Easily done - I (accidentally) left a screwdriver under the floor - I now check more thoroughly. Rudder cables and elevator control runs under the baggage area floor of the Allegro, but I got lucky - only experienced a rattle. The screwdriver was long enough to arc across the battery terminals, but they're recessed, so, in theory, no opportunity for arcing across the terminals - I don't know what effect that would have, but the battery is uncomfortably close to the fuel tank!
The crew in this accident are both medico's, so would be familiar with the concept of counting tools in and out, but of course, they wouldn't have performed the maintenance on a GA registered aircraft.
Perhaps we shouldn't use our messy toolboxes (well I know mine is!), but a shadow board instead - if anything is missing post maintenance, then it's time to go looking!
Mal