System to avoid vehicle petrol theft!

I am facing a problem of petrol theft from my building parking, so I was thinking a system which can intimate or resist or at least give a buzzer to create fear to thieves!
Basic solution is putting petrol lock on my two wheeler which I did but now thieves are so smart they pull out entire valve so any body have any cheap solution on this like if he touch the valve alarm should raise or if he pull valve there should be indication or such!
but cost will be a great factor as I cant put assembly of $200 to save petrol of $20.
Any one having any suggestion?
I know it look like a funny thing that making project for this kind but it is really important in my area right now!
Thanks in advance!

Have you thought to instead of protecting the petrol to protect the perimeter.

Some IR movement detectors could detect intruders before the theft?
This could trigger a webcam to give visuals?

Is it a shared parking?

Ultimate option is to take the petrol yourself every day so there is nothing to steal.
Or give that illusion by using a hidden tank?

Yes it is shared parking, we are planning to place cctv but initiative are too slow so i guess it will take one more year to implement that!
I was thinking of a IT trans receiver module placing near petrol line which will detect any object coming close to it! and using that I can trigger a buzzer or send RF signal with 433mhz module in my flat! any other suggestion other than this or about this?
I dont understand that Hidden tank concept can you explain what exactly you want to suggest?