I discovered while fighting the Titans in their stronghold that I was able to stand just outside of their detection/reaction range and shoot arrows and spells at them without their reacting. (I think only Rock Blast and Toxic Cloud could reach them though. All other spells with the identical standard range would fall short).

For some reason, it never occured to me to test this out with other monsters in the game. Just tried it out in Land of the Giants. Works with the Titans there but not with the Dragons or Devils. In fact, they seem to detect and start shooting at me slightly outside my party's detection range, i.e. just before the detection frames turn from green to yellow.