What does it mean when a school asks for a missing GRE score that I submitted?
We can't really answer whether the school you sent is specifically considering you, but there are three main things that could cause them to ask for the score (regardless of how/why they said they don't have it). The key is to understand that graduate admissions (at least in the US) is a compound process involving the department and the graduate school.

In order of least encouraging to most encouraging (for your prospects):


The graduate school may have either a policy that they only pass on complete records and viewed yours as incomplete or the graduate school wants complete records for all applicants for book-keeping.
The department did not have the data and wants it before it begins evaluating applications at all. Good side = this would mean the department is looking at it. Bad side = some departments in some fields use a cut-off based on GRE scores to lop off the most unlikely candidates (obviously with exceptions mutatis mutandis)
The department and/or graduate school is making selections and needs to complete files before sending out offers. Slightly less positively maybe they need this as a tie-breaker to figure out the last candidate.


Obviously three would be great, but often the reasons for these queries are much more mundane.