Update: it was a letter from the Damage Recovery Unit asking me to file an incident report, which unhelpfully did not reference any specific rental and/or incident. My previous several rentals were checked in fine without issue.

Called National to see what's going on, and they told me that the Polestar I had returned in PHL last week was missing a charging cable. I told the agent that the JFK manager on duty (where I picked it up) sent me on my way without a cable and said he'd note the reservation, as I was planning to drive only ~150 miles on a full charge. The agent said she'd open a dispute of the claim and I should hear back within 1-2 weeks.