Does your fuel contain a Fuel System Icing Inhibitor (FSII) such as Prist??
If so, then the odour you are probably refering to is that of the diethyleneglycol monomethyl ether CH3O(CH2)O(CH2)2OH, a low concentration (0.15%) substance which prevents condenstion icing in the fuel.
I haven't noticed it myself, but I guess it would smell slightly sweet due to the 'ether'.
For more information on this substance click here:
Page about DGME
FSII
DGME and effects to humans!
Hope this helps!
cl12pv2s