I put -5 into a number field in record 1. I put 5 into the same number field in record 2. I entered find mode and specified -5 as my single find criterion.
I found record 1 and did not find record 2.
Is it possible that your field is not defined in Manage | Database | Fields to be a field of type number? Might it be a field of type text?
my bad, that's exactly it.