Ships only use fuel when they are actually moving, regardless of their speed setting. A ship ordered to move to a location, or orbit a body, still has a thermal signature based on its speed setting, even if it isn't moving. I wish this would change, as it makes managing your thermal emissions a pain.
Also a pain is that splitting or reorganizing task forces resets their speed to max, even if they were explicitly set to something else.
Again, Picket sets the speed to 1 km/s upon arrival, which reduces thermal signature. Just moving there and stopping does not change speed setting, and therefore does not change thermal signature.
Thermal signature is bugged anyway, and ships underway are detected by DSTS at least as if they have their max thermal signature.