Mutual understanding via sharing and interpreting inner states is socially rewarding. Prior research shows that people find Brain-Computer Interfaces (BCIs) a suitable tool to implicitly communicate their cognitive states. In this paper, we conduct an online survey (N=43) to identify design parameters for systems that implicitly share cognitive states. We achieve this by designing a research probe called "SpotlessMind" to artistically share brain occupancy with another while considering the bystanders' experience to elicit user responses. Our results show that 98% would like to see the installation. People would use it as a gesture of openness and as a communication mediator. Abstracting visual, auditory, and somatosensory depictions is a good trade-off between understandability and users' privacy protection. Our work supports designing engaging prototypes that promote empathy, cognitive awareness and convergence between individuals.