"No reason, just random pulling over in a lay by, lots of other cars." Is that what he's told you?!
They can't and don't stop for no reason!
They have to have cause and as the car was "generally" insured and I'm assuming mot'd and taxed? And in a state of good repair and the licence plate not obscured? Then the licence plate checking thingy (the name of which bloody escapes me now 🙄) wouldn't have picked up that particular issue either.
He was likely speeding as that's the most common offence and they tend to focus resources on "hot spots" - areas with high fatality & serious injury rates - especially if during the day on a normal day (ie not a bank holiday)
Although he could also have been messing on phone or not wearing seatbelt - you know him so will know what's more likely.
Think you need to dig a bit deeper.
Quite honestly it sounds like he also was obstructive/provocative in some way eg not stopping as soon as asked OR was speeding massively over the limit.
My brother is a police officer, so of course has done his time on traffic duty, his best friend is now in charge of organising where police vehicles are placed to monitor speeding etc - not sure what the role is called but it's his job to organise the resources for catching dangerous drivers. It's all about the "fatal 4" - speeding, drink/drug driving, mobile phone use while driving and not wearing a seatbelt.
An insurance anomaly of this type wouldn't have shown up until AFTER they stopped him and checked the details.
"If, say, you were in an accident while travelling to a meeting off site, would the insurance company ask you why/where you were travelling at that particular time?" Of course! Because their aim at this stage is to avoid paying out!
"It’s not like he’s been involved in an incident which would lead to a claim under the policy." I think she needs to double check with dh on this, potential awkward/confusing conversation with insurers if he's actually got the points for a separate offence or he's been done for the insurance and something else. Because at some point the facts will be recorded with the dvla and if you haven't kept insurance company fully informed that invalidates insurance too.