Losing diesel? 206sw Peugeot ?

Just had i minor accident denting in search by drivers door. Now 2 weeks later my diesel light came on after i thought i had quarter of a tank. Put £20 in it friday (now monday) and have done 15 miles since then. It didnt even move off the drive during the weekend. So why has the light just come on again. Where has the diesel gone. There is no trace under the car. Where should i be looking. Looking under the car it seems dry around the tank. Any ideas. Is there a way to finc out whether the diesel is still in the tank.