is there a gas present in the Martian atmosphere which could be extracted to inflate a balloon with sufficient lift to support an instrument platform for low altitude reconnaissance.
No, unfortunately there's not a hydrogen-like gas in the Martian atmosphere. However, since such atmosphere is mainly composed of CO$_2$, we can approximate in good measure that any lighter gas could provide lift (if inflated with sufficient quantity of it) enough for the purpose, and as I've seen, Oxygen, Argon or Nitrogen, which are 'abundant' in Mars' atmosphere could be used for such purpose.
Or you could just heat the gas up, that could work as well.
The amount of lift that you can obtain is a consequence of buoyancy, which depends on the difference in weight between the fluid that is displaced (i.e., Martian atmosphere) and the fluid that is inside the balloon. Since the Martian atmosphere is approximately 1% as dense as earth's atmosphere, it would take an ENORMOUS balloon to displace enough of the Martian atmosphere to provide enough lift to float the balloon (i.e., the "skin" of the balloon weighs something), any platform, and the equipment that was attached to that platform, unless the instrumentation and associated equipment was very small. Admittedly, the Martian surface gravity is only 38% of earth's surface gravity, but that doesn't fully make up for the very thin Martian atmosphere.
On additional issue: the balloon would not be neutrally buoyant. It would have to generate a net positive lift to get off the ground. As the balloon rose, it would expand, displacing more Martian atmosphere, and remaining positively buoyant as it did so. This means that the balloon would keep rising until it got big enough to burst, which means that a balloon by itself can't stabilize at a low altitude for reconnaissance purposes.