[arkfreepdom]

I could be algae gas if beside an area where it’s blooming. That stuff is deadly.

[pierrem15]

That's a good guess. There are some lakes in Africa that are so charged with CO2 that sometimes they boil over and the gas slides silently downhill and suffocates an entire village.

If there were a pond or lake emitting CO2 or some other non-stinking inert gas it could displace the air and they'd have just walked into it and a couple minutes later collapsed, just like an enclosed space death.

[Getready]

My guess. The algae produce toxins and could have been upstream from where the dog and them drank. It was a hot day.