A sweet smell near the engine bay, a gradually dropping coolant level with no visible puddle underneath the car, or a visible stain or seep around the lower edge of the cylinder head — all point to coolant escaping externally at the head gasket, a specific and well-documented failure mode distinct from internal coolant mixing with oil.
The head gasket seals the joint between the engine block and the cylinder head, and on the engines below, a specific gasket material proved prone to degrading enough to let coolant seep out externally — not into the oil or the combustion chamber, but simply out of the engine, typically from the lower edge of the head. This is mechanically different from (and generally easier to diagnose than) an internal head gasket failure, but left unaddressed for long enough, the resulting coolant loss can still cause the engine to overheat.
This is one of the most searched, most discussed single issues in the used-car world for the engines below — in at least one case, it's become so closely associated with a specific engine family that it defines the entire model's used-buying reputation.
This symptom shows up across several unrelated engines and drivetrains — each has its own root cause and its own full writeup.
This is the single most important check on any car with a documented history of this issue: ask whether the gasket has been proactively replaced with a revised design, and get documentation if so.
Check for a sweet coolant smell and inspect the lower edge of the cylinder head directly for seepage or staining — don't rely on the coolant reservoir level alone, since a slow leak may not show up on a single check.
If buying a car with this issue unaddressed, budget for the repair as a near-certain near-term cost rather than hoping it won't apply to your specific example.