Sorry for the (probably) noob question, but when running in WvW and dropping supply traps ( https://wiki.guildwars2.com/wiki/Supply_Removal_Trap ). I realize that you can only place one trap at a time and that if you leave the map or place a second trap it de-spawns the first.
My question is how do people track when it's "Safe" to drop a trap, there doesn't appear to be a buff or notification that I can see when the trap is sprung (or to show that there is still a trap in existence). When I've asked around it seems a lot of people end up dropping two traps - both a marker and a supply just to see the dots in the area so they know that the trap has triggered (although this seems to require you keep checking map).