Dog Tags are collectible objects you can find throughout the missions in Aliens: Colonial Marines
They belong to Marines who were probably KIA, such as PFC. M. Moore, PFC S. Trubetskoy
Dog Tags List
PFC. M. Moore
PFC S. Trubetskoy
Cpl Cynthia Dietrich
Sgt Z. Forcher
PFC Trevor Wierzbowski
LT. B. Norton