Using LOTS of fuel if left sitting with engine off!!

Hi people... 2007 VVT Auto Jimny, 110000km, 2 inch lift 215s, snorkel... I normally get around 10km to the litre which is pretty good considering the mods. But recently I have noticed that if I do not drive it for long periods. Like a week or so. When i get in and look a the fuel gauge it can be down as much as 1/5 and it has not been anywhere. i cannot smell any fuel where it sits. Although it does get parked on grass only. I use it primarily for 4x4 and to get up and down the extremely difficult hill that my house resides on. So it is left at the bottom during the day as I drive a very efficient rice burner for chores and work. If I fuel up and go for a long run I get a good 370-380km out of the tank but if left just going up and down the hill... 300m each way and a few shorter drives in between I am lucky to get 200 km out of it. Should I get the tank pressure tested or get it on concrete and give it a good looking over?  Thought I would ask here first. Any help appreciated. Cheers, Todd

Logically, if the fuel is going down and it isn't being driven:

-it has a fuel leak or
-someone is siphoning it

Do you always park in exactly the same position? It's not impossible when parking facing uphill to appear to have more fuel than when facing downhill?

Also very short runs like you described will chew fuel.

When you fill up do you record the milage and litres, then work it out , or fill out when the gauge reads low....I ask because our gen3 fuel gauge takes ages to get up past 3/4 , In fact a quick glance it looks like it needs refuelling ......
But working it out , it's consistently the same mpg .

Just going to agree with everyone above.
you are using it or leaking it.
Its got to be worth taking a look underneath for damp patches around the fuel system. The odd smell of petrol is usually a giveaway also, as when working the Jimmy simply does not smell of petrol.
After that as, Fossie said, the gauge has an inclination for reading low. I always filled up when the gauge said empty, so imagine my surprise one day as a low fuel light illuminated even though the gauge was on the empty stops.