All was fine with the car until I took it in to a shop due to A/C
leak. They said they found a corroded O-Ring somewhere beneath the
battery that had been ruined.

So, they fixed that, and A/C is working great, but tonight while
driving (100 miles after picking up the car) the battery drained out
while I was driving.

My suspicion is that the A/C guy unhooked something that connects the
alternator to the battery and didn't hook it back up when he was done.

For reasons of convenience, I'd like to find the problem myself rather
than take it back to the shop, if possible.

Any ideas what someone might have undone in the vicinity of the
battery to do that service that would disconnect the charging system?

Thanks much in advance for any help,

Joe