Hmmm........
Bad petcock possibly?
Don't have a solid answer for testing one of these, as I have never looked at mine.
Leave it on all night the next time you DON'T have to ride to work in the am, and before you leave see if the bike fires up, or floods.
Guess the petcock COULD be dumping too much fuel...but wouldn't you experience a totally different running experience thru the operation?
I DON'T think it would have been battery related, as it did have enough juice to fire it that evening, right? Riding far enough to work to charge it? And still have enough juice to "fire right up" that afternoon? could be possible, but unlikey. When a battery starts to go, it will go down after a night or two. Check your date on the battery.
Leave it set for a couple nights with petcock on. Try to start a couple mornings later when dry (if possible). That might be enough evidence for me to say--Battery is probably ok, petcock probably not a problem.
If it rides good for a week or so after that, and no rain---then wait until dry day again, start it up. After it cools, put your garden hose on MIST and spray the bike down well. See if you have any problems starting then.
So many things it coulda been.....
Ignition switch might have an issue
Petcock
Moisture
Electrical system (lots right there)
Water in fuel
Trash in carb
Compression fluke
You get the idea. I'd just start making good notes of what you have tested to your satisfaction, so you can kinda rule that out.
You know, basic mechanical stuff that we all forget when faced with a problem on our OWN bike.
Remember, fuel-air-fire-compression
Fire and compression are the hardest to test for, IMO.
Hate working on those ole' diesel farm tractors.......ugh.
Good luck and be blessed,
Farmer