Unfortunately in the process of removing the mirror glass I broke it 
I then unplugged the 3 wires connected to the mirror glass and removed it so I could drive the car whilst waiting for a dealer fitting appointment of an expensive replacement. Once disconnected I noticed the BSM malfunction light was showing in the dash - first time I'd seen the light in 7 years of ownership from new - and the driver's side door mirror's BSM lights no longer worked. The chances of the mirror glass lights not working at exactly the same time as the mirror was broken due to something else in the BSM system, such as the radars behind the rear bumper area, must be so infinitesimal as to be considered zero.I assumed that when the passenger mirror was replaced and wires re-connected that the BSM function would work fine on both mirrors. Unfortunately on the way home from the dealer on the first dual carriageway some way from the dealership I noticed the BSM function still wasn't working and the tiny dashboard light was lit. I've now re-booked the car in for the dealer to check out the problem.
Has anyone else experienced problems with the BSM malfunctioning, especially if they've had a door mirror glass replaced? I'll update the resolution for my car when it's sorted.
Update: Embarrassing user admission. I got in the car to go to the dealer this morning and I thought I’d recheck the buttons on the dash hidden by the arm of the steering wheel. Found one I thought I’d never pressed before but obviously I had by mistake when resetting the tyre pressure monitoring just around the same time as the glass breaking action had taken place. Pressing the button turned off the dash light
I did go then on a run along two dual carriageways and was very happy to find that both the mirrors showed the BSM light when relevant 
So a user error on my part and I’ll know that in future. I still partly blame Mazda for the stupid button placement - why aren't they located more centrally under the hazard warning switch perhaps - and thus liable to mean the wrong one is easily pressed in error. Plus I'm trying to understand why a driver would want to switch off the BSM feature. Oh well at least I didn’t have embarrassment at the dealer
