Fosse or no fosse?

[allanb]

Our house was extended many years ago, long before we bought it, and among other changes a shower room with loo was added.  We have seen some drawings found in the architect's file which show drainage to a second fosse septique.  However, they are not proper working drawings, more like sketches, and from certain other details we think they may be just a kind of statement of intent ("how would it be if we put one of these here?") 

The architect himself has gone to the great drawing-board in the sky. 

We have contacted the former owner; he doesn't think there was a second fosse, but it's been a long time and he isn't sure.

The loo isn't used much, but there have been no problems; everything seems to work.  However, we have a routine SAUR inspection coming up, and anyway I'd like to know what we've got.  Can anyone suggest a way of finding out whether there is a fosse, and if so, where it is?  I've done some tentative prodding with an iron rod, but if there are drains down there I don't want to break something.