Check the sender unit and return flange seals as well as the small blanking plate on the top/front of the tank.
I had leaks from all three when the tank was above 1/2 full.
If those are all ok it is likely to be a rust pin hole.
Those can be sealed be cleaning and coating the tank internally, see the Permanent Painted Coatings web site. Sorry, I couldn't get the link to work.
Phil B
