Not strictly a "this is why it's failing" answer, but there's some good clues here.
The following is from Firebug:
The response is:
My immediate question/thought/suspicion is that either value for "layers" is wrong, or the value for "searchFields" is wrong. You're definitely not getting data back, so it's not client-side code not dealing with the response correctly.
If you start here with your own service definition, and use
5 for the layer,
PIN for the search field, and
102113 for the spatial reference you get a little more info. I searched for
54A, and got results back... kind of. Specifically, I got 83 results all with a FeatureId of -1. Does the underlying layer for layer id 5 have a FeatureId field?
Edit: Duh. I could have looked at the specification. Yep, there's an ObjectId.
Further: The query functionality is fine. It's only "Find" that's failing. That's genuinely strange. I'd be tempted to suggest rebuilding / republishing the service - I don't know if this is based on MSD or MXD, or what the underlying data sources are, but there's something interestingly confused.