Bike left in the rain. I'm not freaking out.

It will be ok.  Bikes can handle it.  Just lube it back up.  I have to flip mine over and let the water drain from the seat tube.

Check anywhere there's a screw or nut.    New bike this year and twice this summer at races with Saturday bike drop offs it sat overnight in pouring rain in South Jersey.  By the ocean, awesome salt air!  Both mornings on race day I dried and lubed everything with success, no rust.  So I thought.  Missed the inside of all the bolts(where your hex wrench would go) on the cockpit so they are a bit rusty in there.  One race was pretty much a hurricane the night before.  About a month after when I changed tires to use the trainer there was about 10 oz's of water in wheel.  Never knew that could happen.

Just wipe it down and do normal maintenance you should be doing anyway regardless of whether it rains or not.  Rain is fresh water and the bike was on the roof of your car.  You should be far more worried about when you actually ride through rain as the water that gets splashed on your bike is no longer "fresh" in terms of the dirt, sand, grit, etc that gets thrown up.

If you live in an area near the coast where there is salt in the air and rust is a factor, again...wiping down your bike and lubing should be part of your normal routine.  It doesn't mater if it rains or not...salt is still in the air.

Only thing to add might be to pull the seatpost and let it sit (upside down) overnight to let any trapped water dry/run out of the frame.

Side note to everyone with a carbon bike: buy a torque wrench.  You already spent the money on the bike, protect your investment

Just know that WD-40 is essentially a degreaser.  It will clean your chain, but at the same time remove any of the chain lube that was previously there.

I find WD40 is good for cleaning parts of the bike that I don't want any lube on...such as the chain stays, stem bolts, etc.  But I don't use it on my chain.  I use Rock n Roll gold.  Makes cleaning and lubing the chain extremely easy.