I can now conjecture readily that this streak of light was, in all likelihood, a gleam from a lantern carried by some one across the lawn.
2014 Sept. 23, A teacher, "http://www.theguardian.com/education/2014/sep/23/choosing-primary-school-guide-teachers-guide-for-parents Choosing a primary school: a teacher's guide for parents", The Guardian:
If your prospective school is proudly displaying that "We Are Outstanding" banner on its perimeter fence . . . do bear in mind that in all likelihood it has been awarded for results in those two subjects, rather than for its delivery of a broad and balanced curriculum.