Just wandered into the websleuths thread and found this post:
"Use of their Canon SX270 camera

The recent Telegraaf article (Saturday October 4th) based on the Dutch Forensic Institute (NFI) report concerning the contents of their backback quotes "involved sources" thus:
The final seventy of the 133 sequential photos "appear to have been taken from a deep, dark location, almost certainly after sunset and presumably featuring a large amount of overhanging vegetation." The photos were taken "on April 8th during the night, presumably to draw attention."
That matches the already documented period of night photography between 01:00 and 04:00 on Tuesday 8th. This happens to be the first night out for a rescue team, said to have used light and sound signals. These are not described in detail, but a rescue party would normally use a Very pistol to send up a series of white parachute flares to indicate their own position.

It seems plausible that the women saw the light signals. How could they answer them? Apparently they didn't have the standard orange plastic whistle hikers usually carry. Wonderful piece of kit - costs little, weighs grams, makes a hell of a lot of noise. But tropical mountain woodland won't carry any sound very far, even in the almost windless conditions prevailing that night. And they did have a light source; their camera's flash.
But they couldn't aim that flash directly at the rescue team. Too many trees in the way. They were somewhere near a narrow river, with tall woodland on both sides. But they could see a patch of sky. Did it make sense to flash roughly vertically upwards? The reliable source of historical hourly weather data (links below) skips these particular hours, but highish humidity and partial cloud cover are on the cards. Perhaps they fired the flash a couple of times and saw some Tyndall scattering - like you would see a searchlight beam in damp or dusty conditions. In any case, they had nothing to lose and would certainly fail if they didn't try.
So they tried for hours. But the fingernail-sized flash of their pocket camera was too weak, or there wasn't much scattering and all the light just disappeared straight up. The dark frames themselves show no significant reflections. This is perhaps the most heartbreaking part of their ordeal."