A new update provides some more interesting color... and ample food for more conspiracy theories:
I think you are forgetting that this mission was authorized by the Obama administration last year, that's why USAF Colonel Jack Fischer (whose background includes time spent at the Space and Intelligence Capabilities Office) was placed on the ISS ahead of schedule. He was supposed to be on the Expedition 52/53 crew, but NASA made an announcement in November of 2016 that he had been "reassigned to join Expedition 51/52 in April of 2017", which conveniently placed him on the ISS just 2 months prior to this "secret" rendezvous. Now, whether the rest of the ISS crew had any idea about the mission, I don't know. But I'm certain, with his top secret clearance and "need to know", Col. Fischer was briefed on this satellite's maneuver.