You might look at Susan in disbelief in this moment, BUT...

For one, she was startled. Heart pumping, full of adrenaline, people often don't act rationally.

For two, have YOU ever suddenly had wings you weren't aware of on your back?

For three, can YOU see your own back clearly without a mirror?

For four, you would TOTALLY think there was a bird on your back?

CASE CLOSED.