Even though someone may not look obese on the outside, they can be metabolically obese on the inside. Some people with a high metabolism may choose to ignore their lifestyle assuming they are already healthy, but in fact there may be fat surrounding their organs which puts undue pressure on the body. This could lead to complications like diabetes. People who are MONW (metabolically obese, normal weight) tend to have a higher mortality rate than diabetics who are obese.
Some causes of being MONW include ingestion of artificial sweeteners which trick the brain into thinking sugar is on the way (this causes the pancreas to secrete insulin which is the fat storage hormone); low-carb diet (body burns muscle tissue in place of needed glucose and it's not sustainable in the long run); lack of sleep (increases the level of cortisol which promotes overeating, muscle loss, and gain in abdominal fat), performing all cardio (like running) with no resistance (lack of muscle can lead to insulin resistance).
If you feel you may be at risk of being MONW, dump the artificial sweeteners, get some sleep, balance your diet, and start picking things up and putting them down along with doing your cardio.