It would be helpful to provide an example screenshot or search syntax so we can understand how you are performing the search. Also, are you trying to find a specific package number, or find all the incorrectly formatted ones?
Because your specific field is indexed, you cannot search for the dash directly, but you can search for cases where some special character (including spaces) is between the number.
To search for anything of the format ###-###-##, you could use wildcards as follows within your specific field:
"[0-9][0-9][0-9] [0-9][0-9][0-9] [0-9][0-9]"
Which says "any digit 0-9" in a pattern of 3,3,2 where they are separated by any non-indexed character (examples: @ , ; l - +). This should successfully return all the parcel numbers formatted that was, like 123-456-78 and 456-123-78.
This would also return things like:
- 123.456.78 (dots instead of a dash)
- 123 456 78 (spaces instead of a dash)
- 123.456@78 (random special characters instead of a dash)
That said, I'm confused as to why a field with a standard numerical format is (1) a text type and (2) indexed. Using either a number field with constraints or an integer field would help ensure uniformity of data entry.
At the least, making the field non-indexed means you can search on some special characters such as the dash. Changing a field from indexed to non-indexed is a simple step, so I'd recommend starting there if you can.
If you want to convert over an existing field, there are a few ways to do so and I think it has been covered before here on Answers.