Just a wild thought - Martians take a bunch of capture humans alive, strap a timed or signal release on some viral canister on them, and set them 'free' towards towns or human lines. The surprise element would be lost for the next batch, but I could seem them taking Gatherers with their boxes filled with these 'walking bombs' and just leaving them about anywhere with a timer or activation switch with a sound sensitive setup. Needless to say, any surviving human forces that could report these traps would mean any such box being shot up. I could see these being left behind in town ruins as the Martians pulled out as well. Gads, hope this doesn't 'go viral' (LoL). More pain meds!!