It is very difficult to syphon fuel out. Most cars now have 'anti-syphon'. Don't know the details, but it is very difficult to get anything to go down the pipe, or to get the fuel back out.
People who steal fuel, don't bother with syphoning these days. A sharpened screwdriver jammed into the tank will puncture it, then they collect what they can in a bowl, and don't care about spillage. Once they've got what they want, or can carry, they just leave it to flow out until empty.
There is a locking cap available for Panda. The Fiat part number was posted on here some time ago, maybe more than once. If I can, tomorrow, I'll search for the old box in the garage, and post it again here.
Edit: Found my post from October 2013.
"Locking cap for Panda is 71802520"