Quick hypothetical about fuel leaks?

It depends on where the leak is. A pinhole in a fuel line wouldn't set a code nor will some other items. Smelling it while driving is usually something under the hood leaking.

I don't smell it from inside the car. Just when I exit the car after its been run for a bit.

OK, that could be pretty much anywhere in the lines then. You would need to inspect all of the lines and look for an area where they are damp.

Apparently the metal frame fuel lines were corroded and leaking. Should be repaired now. How are leaks detected in garages? Send pressure through entire system? Usually pretty accurate for finding even small holes?

That is typical. They use a galvanized steel line but then route them close to the frame or other items and that creates pockets where dirt and moisture collect and they rust out. Have replaced thousands of feet of fuel and brake lines over the years. Normally the leaks are not that hard to locate. You put the car on the lift or slide under it and look for stains or moist areas. Some vehicles are well known to have issues in specific areas so you look there first. Like the S series Blazers where they have a bent spot on the frames just in front of the rear tires. GM ran the fuel and brake lines in a cluster in that spot. They collect mud and dirt there and probably 90% of leaks happen in that spot.
If you have something like an EVAP line that only carries vapors you can use a smoke machine to pressurize the system slightly (less than .5 psi) and use soapy water sprayed on it, or look for the smoke. There are a lot of ways that work and you learn over the years. Then you learn bending and forming new lines out of steel, stainless or these days NiCopp line, or buy one of the kits that is available out there.
As for accurate, a modern leak detection system can detect holes down to .03 inch (almost 3/4 of a millimeter) and set a code and the test gear we use can find even smaller ones.

Even with a small hole(s) that don't trigger the CE light would someone smell the gas leaking? And is such a leak a likely big risk to keep driving car for a couple days? I basically noticed smell Friday and had to work all weekend and couldn't drop car off until Monday. I would like to think the chance of starting car on fire was low.

Any fuel leak carries a danger of fire. A leak in a fuel line won't trigger a CEL as they are not monitored. If it was an EVAP system leak, that might trigger a CEL depending on where it is in the system. It sounds like you caught it early.

I was looking into newer Elantras. I guess they have a hybrid version now. How do hybrids work? They use less gas but don't necessarily need to be plugged in? Where does the extra energy come from?

Generally we only answer repair questions. There is a lot of info out there on hybrids and their operation. One item though, there is no extra energy involved.