@GoatGirl19- in the two southern schools I’ve been at, it happens with alarming frequency. Very often, it’s by students who may not know or who have forgotten, but the female professors who have shared this trend with me have never heard their male colleagues referred to as ‘Mr.’ A male engineering professor shared the same observation that his female teaching colleague who was also the dean of the engineering college was often referred to by students as ‘Ms.’ It seems that students are more willing to assume that a male (or male-presenting) instructor has a doctorate, and more willing to assume that a female (or female-presenting) instructor does not.
I’ve seen it happen before and I’ve heard of it happening from female professors (at multiple top research Unis). The other thing I’ve seen happen more than once is envelopes addressed to Dr & Mrs when they both have PhDs. (I’ve never seen or heard of a male PhD incorrectly listed as Mr when both he and his spouse have PhDs). Purpleacorn’s assessment is on-point.
It’s things like this though that make me wish all professors/instructors would make it clear what they want to be called. I hate being called Ms. X (or worse- Mrs. X) so I tell my students to call me by my first name. Most of my professors make it clear on the first day what they want to be called. I know it’s something a lot of students stress about (especially when constructing emails) and it seems like an easy way to make their lives a little easier 
I had a few professors in college that did not hold a PhD, and they were all fine with Mr./Mrs. I don’t think I ever heard a student address anyone as Professor. But I guess everyone has their own preference. However they introduce themselves the first day is how you should address them.