I don't think it has ever really happened during my normal physicals. In my experience, pre cum doesn't develop unless I have had an erection for a while - at least several minutes. Even though I frequently developed erections during my physical exams, they usually did not last long enough to cause me to develop pre cum.
Until - I was in my mid 30's. I had sort of a hybrid real-play exam. I was working a job for a week with a brand-new physician assistant. It was a company clinic that we were staffing together, but it was pretty slow. Early in the week I had made up a story about how I needed to get a physical exam soon. That was the end of it for a few days, until near the end of the week she asked me if I still needed my physical exam. I saw my opportunity - and it was a slow day - so I said yes.
As I said, this was a brand new PA. I don't think she had done many physical exams before. I was pretty excited as I walked into her exam room. I said something like, "OK, I'll go ahead and get undressed." She responded with, "No, you don't need to." Hmmm. Not what I was hoping for, but -- OK.
Since she was new, her physical exam was very slow and very thorough. I would say it was at least 30 minutes. I was fully dressed the whole time but very excited and fully erect during the entire exam, but my erection was hidden by my baggy pants.
She proceeded to check off boxes on the physical exam form, and had checked off everything except 'genitals.' She looked at that for a second and said that I could get Dr. Blank to check my genitals. I told her that wasn't necessary, that she was a PA, and she was allowed to examine them. She said OK and began to put on a pair of exam gloves. I turned away from her and pulled my pants and underwear down. Before I faced her, I said, "Uhm. I think I'm going to have an erection." She just said, "That's OK." I turned around and faced her. I had been erect for so long, the head of my penis was covered in pre cum and their might have even been a drop glistening right at the meatus (opening). She didn't say anything, but I felt the tension in the room go way up. She just grabbed a tissue and wiped my penis off. Of course the pre ejaculate kept coming back.
That was about it. Her genital exam probably took one or two minutes. I didn't ejaculate. Nothing else happened and we never spoke of it again. She filled out my exam form and gave it to me.