Dissapearing fuel leak ???
 

Last night I started the Boxster after sitting for about two weeks. While it warmed up, a sizable puddle of gasoline formed under the engine area.

Went for a drive (didn't notice the puddle right away), noticed the smell of gas, turned around down the street. When I got to my parking spot I noticed the puddle. While I stopped to check the puddle out, another small puddle formed under the same area, so gasoline was leaking all throughout the warmup and the short drive.

So tonight I go to look for the leak. Started the car, nothing coming out from underneath. Looked into the engine while running, no visible leaks. Drove around the block, revved it a few times, let it idle, nothing. Bone dry. No fuel smells either.

I'm stumped. Any ideas?

What year is the car?
The fuel lines run under the car from the fuel tank (under the battery) back to the engine.
There are a few connections in those lines. If you have leak at a connection the fuel will run along the under body pans.
If you have a 2000 or later year model the fuel filter is under the car.
So added connections that could leak.
You need to drop the under body pans and do a good visual inspection and find the problem.
Driving the car could be dangerous if your getting raw fuel back towards the engine you have hot exhaust and catalytic converters that could trigger a fire.

It's a 1999 with a 996.1 3.4 engine.

That's where will be looking first. But regardless where it's leaking from, the weird part is that the leak simply went away. Today I drove it again and it was fine. No one can explain it...

Your car year is one with the fuel filter on the under side of the car.
You can remove all the under body panels then just turn the key to the on position.
The fuel pump will run and pressurize the system....no need to start it.
Then you can go over the system and check for leaks.
Use a flash light and look for fuel stains on the under side of the car and the body panels.
The fuel system is under pressure when running so fuel can spray out when there is a leak.
Even if it is no longer leaking perhaps you can identify where it was leaking.
Let us know what you find.