Welcome to Flyertalk and welcome to the BA forum.

Unfortunately I have seen this happen a few times, and my usual advice is to ask for the bag to be placed on the opposite side of the aisle to where you are sat. That way, even if you are some way from the bag you can keep an eye on it, which you can't do if the bag is on the same side of the aisle (or even above you). The worst case involved someone's passport being in the bag and the passenger refused to get off the aircraft unless the bag was returned to him. It was a bus transfer so actually this did result in him getting his bag back.

And even more unfortunately you won't be able to do much other that hope that someone returns the bag to the airport - they don't always do this - or contacts you direct if they find some contact details. This one is best handled by insurance, and may need a police report depending on the insurance conditions (and is do-able online). Strictly speaking, if a BA agent was the last person to touch the bag, then it is covered by Montréal Convention (MC99), to a maximum value of £1400 approx, but I suspect BA won't be helpful on this one.