The problem with BA's decentralized system is that there is no database. Thus, if the iPad was not noticed (dark case in dark interior) and went on to the next station, it may be sitting in LHR or somewhere else and there is no way to know, other than tracing the aircraft's movements and calling to each location.
Even the free version of FR24 will give you 30 days of aircraft movement.
Nonetheless, I would telephone and not email back to HKG and see if you can't enlist someone to take a look. Unique identifiers will be most helpful as I can't imagine how many iPads there are. Bear in mind that if the iPad went back to LHR, it would likely be logged in under that flight number.