Unregistered, no default service, and inside the Kubernetes NodePort range.
31038 is in IANA's unassigned 31030–31336 block, with no service name, contact, or reference. Nmap's table carries it only as an unknown TCP port with a low observed frequency and has no UDP entry for it at all. Like the rest of 30000–32767 it can be handed out as a Kubernetes NodePort, which is the most common structural explanation for finding it open. Beyond that there is no software known to choose it as a default.
Expose with care
An unidentified listener — fingerprint the banner and judge the service, not the port number.
$ nc -vz -w 3 TARGET 31038Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.