Both Uses of

glimpse
in
The Mysterious Benedict Society

Then, looking behind him, he glimpsed two flashlight beams passing through one of the other rooms.†
p. 109.3

glimpsed = briefly saw

Because S.Q. had glimpsed the note, Milligan had chosen to be caught.†
p. 409.2 *

Definition:

a quick look or partial understanding

The exact meaning of glimpse can depend upon its context. For example:

"I caught a glimpse of her when I was walking to class." -- a very quick look
"I glimpsed at the headlines." -- looked quickly
"Reading the book gave me a glimpse of the life of a devout Muslim immigrant in America." -- a quick, incomplete view