Capture legitimate huge increase/drop in fuel level from noisy measurements
Presumably, fuel theft and re-fueling require the vehicle to be stopped and its engine turned off (?). Every time that you start the vehicle up, re-set the system to be tracking the fuel level in the tank at start-up. Comparing the "fuel level at (previous) shut down" to the "fuel level at start up" could tell you what is going on.

Sloshing can persist of course, even during brief stops. Adding a low pass filter / integrator to the sensor would improve instantaneous readings.

It would be interesting to obtain the Fourier Transform of the fuel level sensor signal while the vehicle is moving, provided that it is sampled quickly enough. Depending on your specifications for fuel level accuracy, you might find that a simple low pass filter / integrator might be equally effective in "dampening" the sloshing effect on the signal. This is how common "anti-slosh" modules work anyway.

Hope this helps.