Keep an eye

The woman was watching my husband all the time while she passed. What  was she thinking. Maybe: "I should keep an eye on that ruffian, maybe he will attack me".
Or  "That nice young man might be my toyboy, I should flirt with him". 
But then she saw me, an odd fat woman who was faffing with her camera and thus forgot all her thoughts. It was me that she needed to keep an eye on.