Back when I posted this thread and didn't get answers on it, I did some digging and came to the conclusion that there are no bass in Pen Lake. The bass symbol on Jeff's Map must have been a mistake. I looked through lots of fishing-related trip reports online and no one seems to have caught a bass there, and when bass get into a lake they usually become quite abundant.
But the most convincing thing I found when I looked into it was this study from 2011 that looked at the ecological impacts of large non-native predatory fish on lakes in Algonquin. As you can see for example in Table 1 on page 340, Pen Lake is actually used as an example of a lake that does NOT have any such non-native species.
And though not quite on topic, one interesting thing to me about the article is that they did their own update of the invasive species present in the subject lakes -- seems the info I've been going on comes from a c. 1990 survey by the ROM. Based on the author's fieldwork, it appears that since then, pike have moved into Galeairy Lake and walleye into Rock Lake, meaning they've each moved upstream 1 dam. Also pike have entered Bridle Lake.
(since we're talking about non-native species, I assume I can name small lakes?)
nvm (to your last post) -- I think Was Hardcore was arguing that there are probably bass in Pen because it's on the Madawaska and there are bass upstream of it. However, I disagree: to my knowledge there are no bass in lakes upstream of Pen. Upstream of Rock on the Madawaska River corridor, yes, certainly there are bass there, but Pen isn't online to that system, it's on the South Madawaska, which I don't think includes any bass lakes.