Park near Harry's apartment
It isn't that Rosa stops paying attention on purpose ... it's more that she does it without really noticing. At least it's still early in the evening when she steps into the park so there's nothing dangerous, but unless someone taps her or she runs into them, she's probably not going to notice them.

She's thinking -- about what she's going to do with her engagement ring and with her engagement in general, about how Harry is and how she should go to see him, about the papers that she has to touch up or finish off.