I've owned three 944/968 series cars - so been there done that with this problem...
It can be a couple of things:
1. Fuel overflow carbon filter is saturated because the previous owners kept the tank full to the brim...
2. You have a pinhole sized air hole somewhere aft of the front seats.
3. Hatch latches may not be adjusted properly - the hatch then doesn't seal corrrectly.
4. The hatch seal needs to be replaced.
Easiest way I've found to trouble shoot this issue is to pull all the carpet up in the rear hatch area, make sure you can see inside the rear wells, and then when it's real dark out, with you laying inside the rear hatch area with the hatch closed, someone with a real bright flashlight shines the thing everywhere around the rear of the car.
Every leak I've found I found this way...
Then when you find the hole, plug it up or adjust the hatch latches or replace the seal.
If you find NO LEAKS, then you have a problem with the carbon gas canister, that's no fun and a ton of money to replace.