My Mustang's battery light came on then went off after a second or two?

I was driving my 1999 Ford Mustang GT yesterday, and twice the red battery light came on for a couple seconds then went off. My car continued to run normally when this happened. I took it today to my trusted mechanic, and he did a voltage test while the car was on, as well as the lights and AC, and it showed about 13.8 V, towards the bottom end of the range it should be. The last three times I've driven my car I didn't see the battery light come on again. My mechanic said my car is fine but that the alternator may go bad in a couple months or so, and that the light may have been a warning as such. In February I had the battery replaced so it is still pretty new. I just want to make sure my car will be safe to drive for at least the next few weeks. I do have to drive about 50 miles to and from a business conference this weekend and I want to make sure there's no real chance it will stall on the freeway.

Added (1). I had the alternator replaced about 7 yrs and about 44,000 miles ago, if that helps.

"I want a guarantee that this old part will not fail for at least three weeks."

The sensible thing to do is replace it now.

I would drive it… Your mechanic is right on the money, and if it does go out, it will still run for quite a while on the battery. Since you have a new batty. Now i wouldnt think twice about it. Wait and see if your alt. Does go out and IF/when it does, deal with it then.

It could have had a little glitch in it and got over it, now its good to go. (I've seen small screws come loose inside the alt. And it made a "short" in it, but didn't last long, kinda like your describing.)

The low voltage cut out won't kick in till the batty. Gets down around 11.2 volts.

Have "trusted mechanic" look again.
Not with just a meter or test instrument - actual eyeballs.
I suspect worn brushes in the alternator. Warning light on/off and then "all seems good" is common symptom.
It's not difficult or time consuming to pull the brushes and check the wear.

Could be a worn alternator. It's brushes only last so long. You can do a few things- clean and tighten your battery terminals to eliminate that fault.

Those cheapo aftermarket alternators do not last long. Get the alternator replaced with an OEM one.

Could be your alternator seven years is long enough to fail. You need to take a bolt meter to your alternator or have a place like autozone test it for free. It's also possible that you have a bad loose connection or even a bad battery that the alternator can't charge.

Bad cable

Your mechanic is correct and the only way to be sure is to replace the alternator now - it could fail today or last months and no one can say which no matter what they think.

That's a sure sign that the alternator brushes are worn out and the spring tension is too weak to keep the brushes making contact on the armature.
Feeling lucky?! There are no assurances other than replacing the alternator now. Think of it this way… If you don't have towing insurance you'll be paying an extra $150 to get the dead vehicle towed to the mechanic to put the alternator in!

If your mechanic gives you a truthful answer all the time, I have to say you need to change the alternator.

The longer you wait it is bad. I would say you need to fix it when you get a chance.