Deer get used to all kinds of things. Equipment running, other farm vehicles, you name it. It's when there's a change in the normal routine that deer will become alert.

A hunter tends to leave a different scent pattern than a farmer going about his business. A hunter's scent can be more concentrated in an area, or may be in an area that humans do not normally go. Either will tip off deer that something is up.

The more you take care of your scent, the greater your chances of not spooking the deer. They may smell you, even after you leave the woods, but if your scent has been minimized by hygene, and additional clothes or other scent control products, sprays, etc., the deer may be bothered.

Bottom line is deer do "understand" that people can represent a threat.