I posted this yesterday, but I guess it didn't go through.

I discovered a little surface rust in the front-driver's-side of the engine bay, forward of the battery. Before cleaning it up, I thought I would ask the forum if they could tell where this rust was coming from.

Thanks.

 

It possibly could be water intrusion from car washes or weather events. If you park it outside there may be a way for water to drip down in from the hood or the cowl.

Check the hood and cowl for leaves and debris, check the rubber seals around the hood.

One factor that could cause corrosion is Hydrogen Sulfide gas from the lead battery off-gassing. It's a dense gas that could potentially settle underneath the battery tray. This could potentially be the cause of the corrosion. It's normal for lead acid batteries to off-gas but excessive off-gassing is usually caused by overcharging the batteries.

 

Another thing to check is if the underbody splash guards and fender liners are intact and correctly installed. There's an amazing amount of water flying around in the wheel well when you're driving in the rain.

As the previous poster suggested, there tends to be lots of corrosion around batteries. When you're cleaning up the spot you indicated I would pull the battery and check below it as well.

 

From the pictures, seems to be surface corrosion. I guess it could happen from humidity in the air, or salt air mist if you live near the sea.