So a 100 mile radius is a rather large area, granted, its at least whatever the answer to pi*r^2 is(31 415.9265?). But if a signal is being sent, it has to be collected right? So the sub is there, underwater minding its own business, whilst another submarine is scanning for any tell tail signs than an object is picking up radio signals, or whatever the signals may be.
When my mobile recieves a signal, my speakers do that weird blerping noise, so wouldn't there be technology in place that could detect the blerping noise (which obviously wouldn't be blerping noise) that would be made by any communications device recieving a signal?
I think the idea of it is for stealth only, otherwise the sub could submerge and pick up the signal, in peace time it doesn't really have much of a use I guess, I mean peace times sub's just crawl around looking for giant squids who want to take over the planet, but in war time sub's won't want to surface so a system like this would probably be used then, and in those cases it would give a fair indication to the enemy that a sub is in the general area of that bouy.
I dunno though, to me it does seem like it would be tracable and usable to pinpoint the location of our nuclear deterrent. I'd be happily proven wrong, in that its not possible to scan for electronic equipment picking up a signal and triangulate where that signal is.