Just a question concerning time and cost here.
The bag was considered low-risk, if any. Four hours into the flight and four hours back equalling at least eight of thirteen hours generating a heck of a lot of costs to the airline with absolutely no return on invest. I’m not a bean counter, but that seems somewhat of the scales to me.
Assuming that this bag was a serious threat to the flight, eight of thirteen hours would have been a high gamble would it not? Hence an immediate diversion would have occurred, disregarding of any duty time or cost.
Based on the info available here today, the decision to return to LHR makes no sense - either it is an emergency and you divert or it is not and you might as well carry on.