An "unwitting participant" refers to someone who is involved in something without realizing it or without intentionally being part of it. They are unaware or unknowingly taking part in a situation or activity. Full definition
In the latest Facebook scandal, millions of people became unwitting participants in large research projects. (psychologytoday.com)
These family members may become unwitting participants drawn into the family drama. (myplaytherapypage.com)
For better or worse, we are now all unwitting participants in what is likely going to be the world's largest behavioral science experiment. (scientificamerican.com)