Probably better for the general public, if they perceive a problem, to initially contact train crew unless it seems rather serious.
My understanding is that the cause of the fire, while regarded as "electrical", is not necessarily a reflection of any inherent issues with the fleet; this was more a one-off with reportedly particular circumstances. I really can't say more as I expect that it will get messy; suffice to say that the facts may come out in another year or so. Sorry; you all just have to wait.
I think everyone until now has assumed that, should a Sprinter catch fire, the cause will probably be related to an engine issue; a ruptured fuel tank is a possibility, but remote. I doubt anyone expected an electrical matter to cause so much fun. Unlike say an airplane, a train can be readily stopped and evacuated if needs be, and the expected fire causes would not likely threaten people's safety.
Drivers do daily checks as part of start-up, as well they usually do have a good look around as practical where opportunity presents (e.g. while fuelling, docking from yard). Drivers don't stick their heads into electrical compartments; in any case these particular ones are locked using keys not issued to drivers. A check of indicator lights, position of circuit breakers and switches; that's about all they do with that compartment. Whether some form of smoke/fire detector is warranted is for others to determine, although given that they don't get keys for these boxes we can assume there wouldn't be much change from what happened at Seymour (fire detected, driver calls for help, unable to open compartment for CFA, fire burns until it gets bored or the CFA can access it).
Unrelated, but anyway; V'Locity trains have a fire suppression system for the traction engines. None for the auxiliary engine, nor for the electrical areas.