Nasa has started work again on a glider for Mars calle Prandtl-M. Preliminary Research Aerodynamic Design to Land on Mars. The project is also a part of th4 Weather Hazard Alert and Awareness Technology Radiatio Radiosonde (Whaatrr) project designed for high altitude research on Earth.
They will be testing the concept by doing a test drop from 100,000 feet where the atmospheric pressure is about 1% of sea level, very similar to Mars.
We already have very high altitude sailplanes that can reach up to 90,00 feet. Airbus II has already had one test flight and is specifically designed to reach 90,000 feet. They are hoping to reach over 100,000 feet with the Airbus Perlan III in 2019.
These high altitude sailplanes require support from wind currents and heat exchanges in order to rise that high. Near surface meteorology on Mars will be key to how long a sailplane on Mars can stay aloft. A lighter gravity certainly helps, but I am wondering how the plane will land.