Russ. ... Good suggestions. I'm sure that the military uses a variety of sensors, including thermal. Likewise, some sheriffs departments with a lot of resources have good thermal sensors mounted on helicopters.

The challenge for SAR drones will be to get these vehicles reliable enough that expensive kit can be mounted on them. Otherwise if the drone goes down, the loss in hardware is expensive.

Another challenge is that every additional sensor adds extra weight to the vehicle. And each sensor also requires power, which adds batteries and even moe weight. It's a feasible problem for UAV airplanes, but a tough problem for small copters where the weight carrying capability is not high.

More work needed. :-)

ASKAR ... Yes very good point. Drones might give clues. They wouldn't necessarily give a guaranteed result - unless they spotted someone waving, or clearly marked distress signals on the ground. On the positive side, if a drone operator thinks he/she has found something, a good approach is to make more overflights at lower altitudes and inspect closely from various angles. Also, the drone could drop a flare or blast a loud horn ... Something to get the attention of a victim on the ground.

Pete