Oh, it's very easy to not know it.
My sister, cousin, and myself have all had stress fractures of our tibias in the recent past. My sister and I had them in the same spot, even, right in the top of the shin under the knee. Both happened after we had started a jogging program.
I'm a little surprised by HOW your patient got it, but that's neither here nor there. My guess is, she at least had pain, but didn't know she actually had a fracture. To make matters worse in that regard, stress fractures, especially there, often aren't seen on plain xray. My sis and I both needed MRI's to see ours. On the MRI they could see not only the fracture line, but a lot of edema in the bone itself (probably because I worked and walked on it for a month before it was diagnosed).
After 4 times of hitting the treadmill at the gym, my knee was hurting. I figured that my poor old joints just weren't dealing well with my decision to exercise, because I didn't do it regularly. The pain just kept getting worse, though, over the course of the next week, to the point that even completely at rest it hurt badly (sitting in my car was torture). I couldn't lift my leg to get in to bed, I had to use my arms to lift it. I didn't know if it was a cartilage or ligament injury, or what (I suspected a stress fracture, because my symptoms were so similar to what my sister's were when she had hers, but the xray I had at my family doctor's office didn't show it). There was no real swelling, bruising, or anything when looking at it. I had to wait a LONG time to get in to my ortho (he's a busy guy), finally I was able to get in with an NP at that clinic after a few weeks. In the meantime, I had asked my family doc about walking on it, and since the xray was negative, she said it was OK to do. She even said exercise was OK, if I took it easy (ie, recumbent bike vs treadmill, to bear less weight). I saw the NP, then had to wait to get an MRI, then wait for the results. Once they had them, she said STOP walking on it!! There was a lot of fluid in the bone by then, as I said. I was non-weight bearing for a week or two, then had to restrict how much I walked on it. I was off of work for a total of two months, since they couldn't accomodate light duty. They can take several weeks to completely heal.
My sister's experience, which was a year before mine, was almost exactly the same. Her first appt with her family doc, she had a negative xray as well, and they called it "overuse" and sent her on her way.
LONG (sorry) story short....it's possible.