I had two bikes stolen (total over £5000 worth) from the locked tow bar mounted rack on the rear of my van a fortnight ago. I was just dropping another bike off and was only going to be 5 min so I didn’t bother putting an additional chain lock on the remaining two bikes as I normally would. As soon as we were out of sight the scumbags were on the job. A neighbour intervened but was pushed out of the way and they kept going, even while being filmed by another neighbour standing about 5m away.
As it turned out the footage was of little use to the police as it did not show sufficient facial detail for identification (both were wearing hoodies). I’m not sure how much deterrence an alarm would have made in that case - it probably would have drawn my attention quicker, but on reviewing the footage I doubt I would have got outside quick enough - it took them about 45 secs to get both locked bikes off the rack. That said I am now investigating installing a dual channel dash cam which has a notification function via phone. It may not alert me quick enough, but at least I will have better footage of the scumbags. Oh, I also now use the my heavy chain lock every time I leave the bikes...