I was at the State Fair this past weekend and happened to be walking past a food stand that sold chicken items. On the signage and surface, it looked like this would be a healthy place to get some food (certainly better than the corn dog stand next to it).
As I was standing behind the food vendor stand, the breeze delivering the inviting smell of chicken to my nose. At that point I turned around and I noticed the process of cooking the chicken. While it was being broiled, the cook literally was soaking the chicken in butter with a dispenser constantly while cooking the meat.
Now this isn't surprising from a food booth at the fair, but it also is a good example of being aware of food prep when outside of something you self prepped. When preparing meals, the number one goal of chefs is taste, which can entail adding extra fat, sugar, salt, or a combo of these. While there are occasional situations that this is not an issue, you should be aware of it and not expect one thing when getting another.
Be aware and know that even though the healthy option you are looking at (say grilled chicken over a hot dog) might not be exactly what you think you are getting.