Whilst it would be very convenient whilst waiting for your car to be looked at etc, I'd certainly give it a wide birth.
It's definitely not discreet at all, as it's literally in the same building as a car garage. They share the same waiting room and entrance/corridor . So whether you're there for legitimate reasons or not (whatever services are on offer), I'm sure there would be a few raised eyebrows and sniggers from the mechanics.