China Security open a lot of checked bags as a lot of passengers departing Chinese airports seem to pack undeclared DGs/prohibited items in them (I do not know if Chinese airports are worse than other airports for the amount of pax carrying undeclared DGs/prohibited items, just that a lot of bags get stopped at Chinese airports for this!)

I am not sure what the process is at PEK but at CAN Security normally wrap the bag in plastic so the pax can see that security tampered with it. At CAN, assuming you has undeclared DGs/prohibited items, they simply remove the items and inform the airline which pax have had items removed. The pax then has 07 days to get in touch to claim the item back. If he needs more than 07 days he can arrange storage via a dedicated agent. If he fails to get in touch they are declared self abandoned and disposed of.

I would imagine it ws China Security taking a look at your bag!