Normally the only fields covered in knee and chest high grasses/plants would be land in the Crop Reserve Program (CRP) because the fields are left not planted and only mowed occasionally. About the only place you see tall grass is along the road ditches and the edges and waterway's of the fields.
After harvest, a Soybean fields would only be a few inches tall. Corn fields are usually gone over with a stalk chopper to cut the remaining stalk closer to the ground before being baled for animal feed. So corn fields are cut pretty short as well.
Back in the day when corn was picked, the stalks would be left bent over several feet high, but with modern combines the stalks are cut by the corn head, which are then ran through the combine to shell the corn from the cob and the stalks and cobs are cut up into smaller pieces and spit out the back of the combine. Soybeans are ran through the combine and chopped up as well. I posted a few videos below to give you an idea of how a combine works and how short a field is after the process is completed.
About the only fields you see now days that are picked, rather than combined, are owned by Amish farmers and maybe for feed of certain pet animals such as horses, goats, ect. Not sure if there are any Amish living near where the truck was found. Even after picking a few acres of corn, the stalks would likely be gone over with a stalk chopper and bales, so they would be cut short as well.
When driving past a soybean or corn field it is easy to spot deer from a pretty far distance that have been hit on the road and end up lying dead in the fields. And deer are basically the same color so they blend in more with the color of a harvested corn or soybean field. I even see some deer lying in the ditches in the grass while driving by at highway speeds. I'm sure I drive by a lot of dead deer lying in the ditches that can't be seen as well. But I think it would be just as easy to see a dead human body lying in a corn or soybean field from the road or while driving across the field with an SUV or walking across the field on foot or from a drone fling over the field from a fairly long distance away as well. Maybe even more so with contrasting colors from the clothing.
While I agree it may be hard to see a human body lying in tall grass while walking past at a distance, it would be much easier to see a body that has been lying in the tall grass for a short period of time from the vantage point of a drone.
I sure hope you folks enjoy watching today's video presentations:
Shredding corn stalks (alloway flail shredder with john deere 4960)
JMO