I witnessed a rather unusual situation a few weeks ago and I'm wondering if I should've done something differently and whether any lessons can be learned from it.

My family was occupying all of row 3 on short-haul flight. At some point towards the end of boarding another family comes on, small kid screaming, the adults bickering loudly. They get to around where we're sitting and after a bit more arguing (probably not more than a minute after they'd got to that point) they decide that they're not flying and get off. A short while later boarding was complete, but of course offloading their checked was taking a while. In the meantime, a member of the cabin crew starts asking all the CE passengers about whether the bags in the bin are theirs and ends up identifying one which no one is claiming.

I'm assuming that's standard procedure to ensure that passengers who offloaded themselves didn't leave anything on board, however at that point I'm also sure that the bag doesn't belong to that family because it was in the bin before we boarded.I said "I think this was here before they boarded" to the attendant, who proceeded to walk up and down the aisle with the bag at shoulder level trying to find its owner; several PAs were also made. Eventually, since no one claimed the bag, off it went. I'm assuming that's standard procedure as well?

Fast forward to LHR, and the passenger sitting in 4A exclaims loudly: "I just got a text saying my bag is still at the origin airport. What do you mean it got offloaded!". An anrgry conversation with the CC ensues, and despite my protestations my wife starts getting involved, saying things like "He was walking up and down the aisle for minutes". Surpisingly that seemed to calm the passenger down and we ended up disembarking uneventfully.

Would the passenger have got their bag delivered eventually? I'm guessing the text didn't come from BA, possibly ground crew found a nametag or something...

Should I have been more proactive and vocal about my certainty that the bag didn't belong to the offloaded family?

I guess my 2 conclusions are: put your bag in the bins on the other side of the aisle and don't be oblivous to your environment even when it's a comfortable and familiar one.