I grew up in Los Angeles area (Arcadia) and friends and I would frequently drive up to Red Rock Canyon area (south of Death Valley) to ride dirt bikes and dune buggies. After I got married living in central valley of CA (Visalia), wife (who grew up riding dirt bikes and shooting guns) and I continued expanding our desert camping trips to Red Rock Canyon area with Quads and 4x4s (while shooting after riding) along with trips to desert and coastal (Oceano/Pismo) sand dunes using paddle tires. (Good times)
Yes, Death Valley and surrounding areas can get very HOT and stay DRY so maintaining good hydration (Sipping water all day long is key) is essential to survival as dehydration and heat exhaustion/stroke can set in FAST.
Likely anyone who left their firearm out in the open had more pressing issues on hand like serious medical or life-or-death issues that prompted them to "get the heck out" without their guns. Perhaps gun owner succumbed to dehydration/heat injury and became delirious and didn't care about leaving the gun. Just like many Western movies where desert heat fries your brain and you start seeing mirages and landscape moving around IS REAL as even with mild dehydration and heat exhaustion I have experienced made me not think clearly (But if you ask my wife of 28 years, I don't think clearly even on a good day
).