Endoleak is one of the major complications after surgery to treat abdominal aneurysms. It is characterized by blood flow outside the stent graft within the aneurysm sac, leading to increased pressure with a concomitant risk of rupture of the sac itself. As standard screenings are operator-dependent and intrusive, this paper proposes a battery-less method to wirelessly monitor the presence of endoleak-related blood flow. The unobtrusive sensing element is an antenna, equipped with an auto-tuning RFID IC and a portion of bioresorbable coating, placed proximally to the stent and inside the aneurysm sac. The presence of liquid blood induces the hydrolysis of the bioresorbable coating, leading to the exposure of part of the antenna to the biological environment. The variation of the antenna boundary condition is detected by the aforementioned IC giving threshold levels following over time the phenomenon evolution. Numerical simulations and a preliminary experiment demonstrate the robust communication link and the feasibility of the sensing mechanism.