I had this happen to me a couple of times on the MUP when I first started riding. I was so caught up in my own little world that I didn't pay enough attention to what was around me. A few people startled me when passing. Some called out, some didn't. I didn't blame them. It was my own lack of attention and lack of confidence and comfort on the bike. I learned to be more aware of what is around me and got more comfortable on the bike. Once I started the commute, I soon got a mirror and I love it.