Do ball lock gas lines seal themselves?

Sort of a keg noob here...

Last night I racked a beer into a corny and put it in the fridge to cool overnight.

I accidentally left my CO2 valve open overnight and this morning when I went to hook up the gas nothing came out.

I'm trying to figure out whether I simply ran out of CO2 (but only after 5 batches which seems small) or if the gas was leaking out of the disconnected ball lock gas port (do the ball lock ends provide a gas seal if they're not connected to a keg?)

Probably a stupid question, but I figured all of you wise folks could help.

Cheers.

When a quick disconnect is left unconnected it should seal gas-tight, by virtue of the tiny o-ring on the plunger inside the QD.

That said, it's good practice to shut off the gas to a line that isn't hooked to a keg, as "stuff happens" and the o-ring or the seat on your QD may not be perfect.

When you get your tank refilled or swapped, the first thing you should do after you reinstall it is to dunk that QD in a bowl of water and turn on the gas...

Cheers!

A few other things to look at:
QD's can be disassembled. Take a large screwdriver and unscrew the top. It's good practice to clean your liquid QD's when you change kegs, but gas QD's can stand a little maintenance as well.
A shutoff valve on the gas line is a good idea (Or a manifold)
If you're using flare QD's instead of barbed QD's, you need to keep an eye on the fit between the swivel nut on the gas line and the QD. They can leak.
If you do use a manifold, you need to put a washer between the swivel nut and the manifold, as metal on metal will not seal reliably.

I could see a 5# tank running empty after 5 kegs, IF you force carb. That and too much "Gotta purge the oxygen out!". In other words, if your process involves filling an empty 5 gallon keg with Co2 then filling it with beer, you're wasting a LOT of gas. Put the beer in it first, then purge the headspace.

Just a few thoughts...