I'm not aware of any actual data on this, but it seems to me that those who choose not to be vaccinated are probably also very unlikely to be tested on any regular basis (if at all), and also (at least in the UK, where I am) not engage in any kind of voluntary contact tracing.

There seems to me to be a gap between what people think is happening, and what is actually happening. How exactly are people who haven't tested positive for Covid (either through a negative result, or simply avoiding testing altogether) going to be identified as the source of an outbreak if they're also not engaging in contact tracing in the first place?

As it stands, it seems to me like the most likely people to be contact traced are those who are also testing regularly, and are also the most likely to be vaccinated. I would be surprised if non-vaccinated people are anything but rarely identified as the source of an outbreak, precisely because they're also much less likely to ever get tested (unless seriously ill) or engage in contact tracing.