Recent convergence between low-cost technology, artform and political discourse presents a new design space for enabling public participation and expression. We explore non-experts' use of place-based, modular sensors to activate, author and provoke urban landscapes. Our work with communities of bicyclists, students, parents, and homeless people suggests design opportunities for merging grassroots data collection with public expressions and activism. Members of each community were given probes that represent the measurement of exhaust, smog, pathogens, chemicals, noise or dust, and asked to engage with them as fully functional sensors over the course of one week. Our findings offer insights into participation, environmental sensing, and data sharing within and across four different communities, revealing design implications for future sensing systems as instruments of social currency and political change.