Shadow of the Tomb Raider has 16 Challenges but only 15 are needed for 100% Completion (Completionist trophy and achievement). Each region has different requirements and new unique tasks to complete. The tricky part is that the game does not tell you what to do and unlike Collectibles, the Challenges do not get marked on the map. To make this easier, buy the skill "Owl's Wisdom: Reveal Challenge objects while using Survival Instincts".

It's highly recommended you do the challenges on easy difficulty. That's because on easy the Survival Instinct can be used to highlight them if you have the Owl's Wisdom skill unlocked. Climbing paths are also marked clearly on easy. On hard there is no survival instinct and no markings so it would be harder to do.

After the story you can go back everywhere in free-roam, except to Croft Manor. The challenge "Bullseye" in Croft Manor (young Lara in garden) is missable, but it is not needed for 100% completion or the trophies / achievements. It only gives some extra XP and doesn't count towards any map.

Location: Cozumel

Whistle in the Dark
Objective: 5/5 death whistles knocked down