Rule 34 (Manoeuvring and Warning Signals) has several sub-rules that require careful reading. Most of the sub-rules require that “vessels are in sight of one another” before they can use particular sound signals, e.g., one, two, or three short blasts. Also, some of the sub-rules apply only to power-driven vessels (like our vessels). When other vessels cannot be seen due to restricted visibility, your vessel should not make sound signals that require that “vessels are in sight of one another”. This even applies to the “five short” signal where you indicate there may be some danger or that another vessel’s intentions are unclear.
Around Comox it is rare to have severely restricted visibility, e.g., less than a hundred feet or so (but it can happen). In "normal" restricted visibility you are more likely to see a vessel emerge from fog (or a rain squall, or whatever) more than a cable or two away. So RCMSAR vessels moving at slow speed (due to the conditions) should be able to stop within one or two boat lengths and ensure that no collision takes place.
The likely sequence is to become aware of another vessel while it is hidden in fog, perhaps still several cables or more away by hearing a fog signal or by seeing an appropriate target on radar. At this point your vessel should be aware that there is another vessel nearby and take suitable precautions (under Rule 19 and Rule 35). Should another vessel emerge from the fog, then the various sub-rules under Rule 34 can come into play -- you are now in sight of one another. Up to that point Rule 19 (Conduct of Vessels in Restricted Visibility) and Rule 35 (Sound Signals in Restricted Visibility) should keep you safe, and the same for the folks on the other vessel.
Note that the “five short signal“ may be supplemented by a light signal of “at least five short and rapid flashes”. Although the COLREGS do not specify, such flashing light signals should be synchronized with the blasts in the sound signals; one benefit of synchronizing the sound and light signals is that it will be clearer to others which particular vessel is sounding signals for uncertainty or danger when there are several possible vessels in sight.
Note that Annex II 1(b) specifies that the whistles of larger vessels should be at a lower frequency. Thus the whistle of a BC ferry is much lower in tone than the horn of CAPE LAZO (and recreational vessels). Thus, if you hear a fog horn in restricted visibility that is at a very low frequency be aware it could be a very large vessel… yikes!