RAF_Techie101,
I know that the bomb bay firewire runs through the No 7 tank dry bay but this is not dedicated fire warning for the dry bay itself. A bomb bay fire warning, by definition, indicates a bomb bay fire. Had the crew member on the periscope not seen fire, and with no other indications, the crew would have believed the warning to be spurious (albeit unusual).
A dedicated dry bay smoke detection system would give a quicker warning to the crew, buying precious seconds. The potential down side is that, unlike the underfloor bays, the area could not be viewed for confirmation.