IanMunro, that;s what tends to be known as a ‘bailout bag’, designed for vehicle ops for military/PMC use. Don’t want a big bulky load bearing vest on in a Toyota in 35 deg heat, so stick your bits and bobs, mags etc in that and just grab it if you need to get out of the vehicle.
Look ar Warrior ones, sold by UKTactical, or the SoTech Go Bag, think OpTactical do them. Tend to be in manly colours like OD or khaki, and are a bit more ‘burly’ looking than some. If it’s got molle/PALS loops you can customise the sort of pouches it has too. Or, look at Maxpedition for civvy/military crossover gear carrying stuff.
Personally I use a US ‘Claymore’ mine bag, it’s just a simple canvas bag, nice and lightweight but durable and with 2 sensibly sized pockets, holds my phone, wallet, I Pod, Leatherman and Surefire E2E with room to spare for drinks, book etc… Mind you, I am a bit of a walt nowadays….