Does ethylene glycol sink in water? I’m a little concerned that there is a far stronger mix of antifreeze in my car's coolant system than I am aware. I check the antifreeze level in my car (using a refractometer) when the engine is cold (after being left overnight).

During the night, can the antifreeze settle to the bottom of the cooling system, thus leaving a weak mixture of water and antifreeze within the coolant reservoir up top, where I take a sample with the refractometer?
Ethylene glycol is completely miscible with water in all proportions (as the commenters point out). Therefore, once mixed, the glycol will not separate from the water, ever.

The only exception is if it gets so cold the mixture starts to freeze. Freezing will cause some degree of separation as the water will solidify first. But if it is that cold, separation will be the least of your problems.