As a alchemist/trapper build, i have a few ideas on this subject. First i have a trap build that seems to work very well for my own IG reps. It is a gator clip anchoring one side of a trip-line, inside the rep, with a momentary switch between the anchor and the exit hole for the line. When the line is taught the switch is down and the relay is open, the line goes slack for any reason and the trap goes off, with a 80-90Db buzzer. As for a hold free system, red cord. Have a trap fitted with a x foot red cord and when it is set off, pull the cord, use that to measure and be done, there is also an element of the honor system when working with traps, i play airsoft and AOEs are almost never measured. It is just accepted that if you could reasonably be in the AOE, it is taken. even better would be to have the cord shot out in some manner for quick reference. As for the usefulness, i had the idea of adding a "formal-ish" system to trap. where a trap master could "invent" a ward like system or mimic other useful effects, at the cost of gold. Also in our chapter, and i am really enjoying this when it happens, traps are taking on the role of breaching and destruction, allowing the players to alter the landscape using that skill.
my 2 copper
Michael H./Gabriel Di Greoni