A mirror can hold a face while the person’s back remains in the foreground. A window can show both a room and a street. Before calling a reflection a symbol, describe how it changes what is visible.
Draw a rough map of the frame
Pause a suitable shot and mark the person, reflective surface and reflected image. A few boxes are enough. Label which information you see directly and which arrives through the reflection.
Ask what would disappear if the reflective area were covered. The answer might be another character, a reaction, or simply a competing patch of movement.
Watch the shot in motion
A still image can hide the reason a reflection matters. Play the scene again and notice whether somebody enters the reflected space before entering the direct view, or whether a movement changes which face is visible.
Record one specific moment rather than describing mirrors in all films at once. Your note should help another viewer find the same evidence.
Let interpretation follow observation
You might read the arrangement as distance, uncertainty or divided attention. Present that as a reading, then explain the detail supporting it. A reflective surface alone does not prove a single symbolic meaning.
For another way to track the limits of a frame, see doorways and unseen space. Both exercises begin with where the film allows you to look.