Requirements to use Detect Magic as a trap detector:1) Standard action every round.
2) The person using it must be in front of the party.
3) The area being explored must not have any other magic.
#1 may slow the group down.
#2 means that if there is someone else with magic on them in front the detector must wait three rounds in order to determine the location of magic sources. Imagine doing that every 3 rounds. We move, detect, detect, detect. Repeat.
#3 is basically the same as #2 but may also mask the aura of the trap.
I know this post from a while ago, but I did not see this pointed out: Regardless of whether there is anything else in the cone or not, it takes three rounds to get to the locations of magical auras (beyond "somewhere in the cone"). If you are looking for magical traps, the location is quite an important piece of information.