What does keep your nose to the ground mean?

I suppose it could mean "keep your head down, " but it's more likely that it's a misrepresentation of one of the following idioms:

"Keep your nose to the grindstone" means to work in earnest; hard and steadily. "Keep your ear to the ground" means to listen carefully, intently; stay informed.

"Keep your nose clean" means to stay out of trouble.

"Keep your feet on the ground" means to have a realistic understanding of your own ideas, actions, and decisions.