Some people call coworkers, friends, or even strangers or customers "sweetheart".
"Sweetheart" is a common way to address your children, your boyfriend/girlfriend, or your husband/wife. But it's less common to use this with other people. When you do, its sounds friendly and a little like you're pretending to be that person's mother, father, aunt, etc.
It's mostly women and gay men who use "sweetheart" with strangers and friends. In the past, men used to use "sweetheart" to address younger women, but now a lot of people think it sounds sexist.