Cache?

None of the above. 

Two clues for you.  1) Western side of the park.  2) If you look closely at the treeline, where it forms a "V" that may help.  Sorry I was so long in responding btw...

Canoe?

Ok, well in the interest of moving things along, there may just have been a bridge that crossed that "V" in the distant past.

Ding ding ding ding ding ding!  AlgonquinLakes has it!  So, if you haven't done this....well, the 2120 from Cranebill to Islet is not exactly a pleasure cruise, and you are rewarded by being on this marshy bit of Islet.  Then five minutes later, you're dealing with this 45 meter up/down, about which Jeff says (in his map), you'll want to plan at least 30 minutes because it is so steep. 

I actually did not see the portage entrance, instead I kind of zeroed in on a log bridge in place for hikers to cross at the bottom of the "V" you see in the photo.  So, I docked up there, unloaded, and made my way across.  I don't know if this was a better path to take than the actual portage or not.  Going up the hiking trail from that little bridge required an s-curve turn, in which it was necessary to bypass a nasty chunk of angle iron sticking straight out, awaiting arms, legs, canoes, etc.  When I did see the portage trail that I had missed, it really is straight up.  No curve, no nothing, just straight up from the marshy side and featureless so nothing really in the way of footholds.  The other side, that heads down to the lakey side of islet....very steep and kind of erody, but I don't think as steep as the marshy side that I missed.  

All in all it was an interesting morning.

OK, Mr. All of Algonquin, you are up!!!