What would be the most likely cause of sudden decreased proprioception as in loss of visuo-spatial depth perception and orientation (cannot walk straight line, bump into curbs when driving/parking, drop objects etc)? no dizziness or wobbly gait?

This could be due to different issues but you need to see a doctor and have this checked. A neurologist is likely the best doctor to see. I would be concerned about a stroke, especially with the sudden loss. But, an imaging study of the brain, like an MRI, should probably be done to find out what the issue is. So, see a neurologist and get testing done. Good luck!

Dr. Ferranti is correct, as a sudden change could most likely represent a stroke, and this needs evaluation ASAP. Other possibilities should be explored. Would STOP driving, avoid activities at heights, and not use power equipment.