I was reading through the history of the order and came across the following
942 1081 Stonehenge: Contact lost with London covenant, which had moved in the city repeatedly. [HtM]
952 1091 Stonehenge: Nova Roma magi report contact with magi of London covenant, in a clash over a vis site. No further contact with London covenant reported. [HtM] London covenant had always been mysterious. Its fate remains a secret, but they left no one behind to care.
So their is precident in "secret" vis sites. Granted this is between 2 covanents but the same should apply for lone Magi