I am in the process of trying to figure this out, but I don't think a spirit can conceal anything from mechanical surveillance.
The timeframe is such that, and yes this is the GM attempting to create a sense of tension/drama/urgency, the team that has to worry about being spotted will just make it, IF they travel by the fastest route at the least conspicuous speed, AND their fuel might not hold out. Cutting across Martell's land will both guarantee the fuel and time will get them there, but also practically guarantee them getting spotted by an extremely well armed person.