My educated guess would be an application using https without a User-agent string that the FMC can parse. The destination is in Akamai's CDN so not much of a clue there. Possibly it is software updates or streaming content.
Iwould check one of your internal endpoints and use something like tcpview to determine what process is contacting that destination address.
If you had Cisco Stealthwatch (or some other Netflow collector), you could use the AnyConnect network visibility module to gather the information.