Thermal imaging won't find someone sheltering behind a boulder or in a cave.
A few points I noticed from this write-up on the trails where they went hiking
https://backpackers.com/spring-hikes-the-maze-loop-in-joshua-tree-national-park-california/
- hike before beginning of June, not during summer, the heat is too extreme
- the trails aren't well signed and easy to get lost
- bring a map and compass and alert park rangers of your plans before you go
- there are peaks that will give great views if you're willing to scramble up them, but make sure you know how to get back to the trail.
- the loop described is quite strenous and would take 4-5 hours.
Since they were supposed to check out of their Airbnb by 11, and considering the mid-day heat, it seems to me they intended a quite brief hike. Unfortunately, that might mean they didn't bring much water.