Will the last electrical receptacle on the same circuit have two cables entering the box?
Then it's not the last outlet.

An outlet is any point of use, i.e. an overhead light or hardwired exhaust fan.  A receptacle is the fancy name for the familiar socket.  A receptacle is one type of outlet. 

And it's not n cables entering the box.  It's 1 cable entering the box and n-1 cables exiting the box to go to downline load(s).  n can be 3 or more, by the way.

Every load must attach at a junction box or fixture that must be accessible without disassembling the house in any way (other than the fixture itself, obviously).  So there should never be a need to unscrew wood panels, bust up and re-mud drywall, etc.  Between accessible points there can only be wires. 

So if you take covers off things, and remove lamps etc., you should be able to find it.   There are electronic tools which can aid in the search greatly.