When picking up a backpack or other container. Any item equipped on shoulder slot (left) moves to backpack. Has occured several times.
When near items, items transfer to backpack.
Being conscious of the issue I have had to regularly check packs and items to avoid loss (which has occurred).
Most recent event varied in that equipped boots transferred themselves to pack while walking, received cut and infection.
Inconsistent but common:
Pickup pack, notice missing weapon, check pack contents, contains weapon.
Re-equip weapon, notice weapon missing, check pack contents, contains weapon.
Check pack for items recently viewed in vicinity which have transferred themselves to pack.
Check pack for equipped items/clothing which have transferred themselves to pack.
Requires constant vigilance to ensure items are not lost which can result in further consequences (lost weapons, items, injuries due to missing clothing), or stamina penalty when finding heavy objects in pack.