Somewhere in your inventory database is a record for a bulletin on the edge of town that came down when the road was widened. Nobody told the database. The record keeps its panel ID, its format and its place on the availability sheet until someone driving the corridor with the list notices a gap in the skyline. Multiply that by every lease that lapsed, every face that moved, every structure that went up on a parcel you never heard about.
So the inventory analyst plans a windshield survey. A truck, a tablet with the face list, a route cut by market and by road, and a couple of people who would rather be doing something else. It works. It is also slow, it depends on the weather, and it only sees what is visible from the road you happened to take. The same drive is how you find out what a competitor built on the other side of the interchange, and it is the only way you get a first look at a market you are weighing, short of a trip nobody has budgeted.