[qclibre22]

On a ketogenic[diet] ... your body uses fat instead of carbohydrates for energy. This shift causes a major spike in ketones, the source of acetone breath. ... Sweet-smelling breath may be a short-term side effect of this type of diet.[0]

[0] https://www.webmd.com/diabetes/acetone-breath-diabetes

So low blood sugar causes sweet breath.

[OkayPhysicist]

Ketones are the "metallic" smell. I wouldn't describe it as sweet.

[jiggawatts]

It's literally the homeless alcoholic smell.