1. Antibacterial Agents Used In Unexpected Products like Toothpaste. Antibiotics are so effective at causing weight gain that farmers add them to livestock feed. Antibacterial products too, particularly those containing triclosan, have been linked to numerous ailments including weight gain. A Stanford study from 2013 showed that higher urine triclosan levels are linked with a higher body mass index. See the study linked: here.
Triclosan is an ingredient in major toothpaste brands like Colgate (though Crest appears to not use it), many soap brands (Dove, Lever, etc) as well as deodorant brands like Old Spice. See a more comprehensive list linked: here.
2. Low Vitamin D Levels. Low vitamin D levels has been repeatedly linked to weight gain, particularly in older women. In fact low Vitamin D levels have been associated with a range of ailments including cardiovascular disease, mood disorders, and declining cognitive function. See a review linked: here. It is such an easy thing to correct too - either go outside more without sunscreen or take a supplement!
3. Eating Too Little Fat. Healthy fats like those in olive oils signal to your body that you are full, on top of the many other health benefits of healthy fats. In studies of equivalent calorie diets, patients who incorporated healthy fats (rather than eating a low fat diet) lost more weight and were more likely to stay on the diet. The study is linked: here.