Somewhere on the drive home from an open house, usually around the second traffic light, an agent remembers the sign on the corner across town. Every agent has done this. It isn't carelessness. It's how the day is structured.
You place signs in the morning while thinking about the showing ahead. Each placement takes thirty seconds and no part of it is memorable. Eight hours of visitors later, the locations of the first few signs are simply gone, and the tired end-of-day brain checks the corners it remembers rather than the corners that exist. Signs on busy roads make it worse by getting knocked over or moved, so even a good memory can be wrong about where a sign currently is.
What it costs
A decent A-frame runs $20 to $40, so a couple lost per season is real money. The bigger cost is the return trip the next morning and the occasional fine when a city or HOA finds your sign standing three days later. One agent's leftover sign is also every agent's reputation with that HOA.
The habit that fixes it
Record the sign at the moment of placement, not later. Later never happens. Then collect against the record: drive the route in reverse, check each sign off, and don't head home until the count is zero. Whether the record is a note, a marked-up map, or an app is secondary. It has to exist outside your head, and it has to be made in the same thirty seconds the sign goes in the ground.
SignFinder is that habit as an app: tap the map when you place a sign, tap the sign when you collect it, and the count of what's still out is always on screen. But the habit matters more than the tool. Keep the record somewhere that isn't your memory.