I am looking for any related work to the following problem. Say you have a large directed graph $G$ and you want to find rare (or unique) subgraphs of minimal size that are not isomorphic to any other subgraph of $G$. I assume this problem is NP-hard but is there any work on it? For example, exact algorithms for sparse, planar etc. graphs or approximation algorithms or even heuristics. Just to fix ideas, every node or edge connected pair of nodes will be isomorphic to some other subgraph so the minimum size this could ever be in a non-trivial graph would be of size $3$ nodes.
There is a very nice bibliography as of 1995 of the subgraph isomorphism problem linked from David Eppstein's publication page . Is there anything more relevant to my problem?