Sudden deflation of a mountain bike tube without a bang?
You say that both tyres deflated suddenly and you pumped them up on Sunday from flat.

I think the sun has caused this in one of maybe two ways. If both tyres were flat on Sunday because they had slow punctures, then when they were out in the sun all that time, they got hot and the pressure increased. That increased pressure made the slow punctures into much faster punctures until all the air was gone.

Or... one or both tubes were fine, but the heat from the "scorching sun" simply caused both tubes to go flat by heating up a section until something gave way. The only surprise is that you didn't hear a bang in this scenario, which would make it seem less likely. 

Lesson: leave your bike in the shade.

In either case, you want make sure that there is nothing sharp left in the set up to cause further punctures. Check the rim strip to see that it is in good condition. Check the tubes to find out where the holes are. This can help narrow the search for the sharp object (that's why people line up the valve with the tyre branding). check the tyre and remove any sharp objects. Replace tyres if they are getting too thin to stop punctures.