Explorer's Satchels are brown leather rucksacks that contain maps to Secrets; if collected, the reveal e.g. Survival Caches on the in-game map. They are similar in function to Monoliths and Archivist Maps.
They are not uncommon but often lie a bit off the beaten path and sometimes require Lara to take a detour to find them. Sometimes Lara will find the corps of a explorer right beside the satchel.