I would pull the carb, you can take it off in just a few minutes. Fill the bowl up with fuel to the correct level and see where it leaks from, if it is leaking. I've seen carb bodies crack and power valves leak.
I don't see how the fuel could siphon back into the tank. The needle and seat are at the top of the float bowl. If the fuel level in the bowl dropped just a little the float would lower and the needle would lift off the seat exposing the line to air. That would prevent any siphoning from happening. I'm not sure but I think the needle and seat are completely above the fuel level in the bowl. Its exactly like your toilet bowl valve. I don't think siphoning is possible.
It does not take gasoline very long to evaporate. Especially now that it has 10% ethanol in it. If it sits for five-days vented to the atmosphere it could just be evaporating through the vent tubes. You could plug the vents and after five-days remove the plugs from the vents and see if starts right up. You could also remove the carb top and see if the bowl is still full. Modern carbs have vents that close when the engine is "not" running so the fuel can not evaporate.
I have a 1966 Shelby GT-350 that I never start. I've driven it 100 miles since 1982. I've been in flathead/hot rod mode for years since I burned out on the Mustangs. Maybe once every two year's I decide to start the Shelby up and take it around the block. When I start the car it immediately starts to flood out because the power valve has rotted away and fuel can flow straight through hole in the diaphragm and into the engine. I now know if I want to start it I will need to take off the front bowl and stick a new power valve in it. I probably get one-mile of driving per power valve. Afterwards, I always tell myself, this is stupid I'm going to start this car up at least once week but then it never happens. Too many projects.
If this was your only car you would be driving it nearly every day and the fuel would not have enough time to evaporate. Ideally carbureted cars need to be started every couple of days. Carburetors do not like not being exercised. We are spoiled by fuel injection. There are no vents in this system. The pressure can be held in the line for days. The fuel pressure cannot drop as long as the check valve in the electric fuel pump holds and the injectors are not leaking. That fuel will sit between the fuel pump and injectors under pressure forever, ready to instantly start spraying with the first revolution of the crankshaft. Its best thing that has ever happened to automobiles. But its not nearly as cool
as a Stromberg carburetor.
If you had a bad check valve in your mechanical pump the fuel might be able to drain back into the tank but that would only be the fuel in the line up to the carb but not the fuel in the float bowl.
A little tip if you have your carb off. If you put a thin layer of white lithium grease on both sides of your carb's base gasket it will not stick to the carb or intake the next time you remove the carb. If your pulling carbs off a lot, you don't want to be scraping off gaskets, its a real time saver. Plus your not tearing up your nice machined surfaces with a putty knife. We rebuilt Holley 4V carbs on the big Ford trucks at work so often that we started greasing the base gaskets because they would get baked onto the intake. You could not scrape them off we had to grind them off with a Roloc metal conditioning discs. You don't want that stuff going in the intake, the abrasive on the discs will destroy an engine. It took longer to get the gasket off than it did to rebuild the carb. Same with the bowl gaskets, we had to scrape them out with a little screw driver.. Those we sprayed with silicone spray to prevent them from getting baked onto the carb. It did not hurt a thing, the gaskets still sealed perfectly and you could rebuild the carb the next time a lot quicker. I was taught this trick from a 1960s-1970s Ford dealership's, carburetor rebuilder. He left Ford to come work with us.
