The mouth offers valuable insights into the condition of the human body. Yet, deploying intraoral sensors to measure oral temperature or jaw movements poses challenges in safety and acceptability. Consequently, real-world data for intraoral research is scarce. To address this gap, we leverage the widespread use of dental retainers and enhance them with Densor: an electronic sensing platform requiring only a standard smartphone for charging and data retrieval using a Near Field Communication interface. Its low power architecture enables prolonged sensing on a single charge, making it suitable for sleep studies. It can provide practitioners with feedback on treatment compliance, and is even able to detect if the user is speaking or drinking water. Densor presents an intraoral, actively powered, battery-free platform featuring multi-modal sensors and an extended lifespan.