I don't know of any attachments that would warn you of a hole as you're moving forward. As someone that frequently finds surprises in the tall grass and weeds, I would just say keep your bucket low in front of you, and proceed slowly. When you see a sudden opening in the thick grass, beware. If it's really too thick to see through, even a few feet in front of you, that's when I really slow down and take it easy. I've never dropped into a hole per se, but I have found a few natural springs that way.
Sounds like it's your land, so once you mow it a few times, you should be able to learn where the pit falls are. Maybe you can mow it more frequently to keep the growth down and the hazards will be easier to see.