How can I detect events on fuel tank
Welcome to SO! It looks like you have a time series problem. Typically the first step when dealing with time series is to consider the difference. Let us define $f(t)$ as the fuel level at time $t$. You would want to calculate $diff_{\text{fuel}}(t) = f(t) - f(t - 1)$.

After this step you will likely see that the spikes that you identify as bad data are outliers. You could detect these by for example looking at all the data below and above your 2.5 percentile or 5 percentile. Typically this requires some careful analysis work to ensure that you do not delete too much. Once you have identified what the best workable percentile is you can could Winsorize your data.

Lastly, you would look at the data points in the bottom of your resulting distribution. These will likely be the points you identify as theft.

I hope this helps.