Natalia purposely kept her eyes partially open, only enough so she could see the HYDRA agents pause only briefly before moving on. But also closed enough that they would think that she was fully enjoying this kiss. Which she sort of was. Bruce certainly had taken her instruction seriously enough that they'd very easily convinced the two that it was just a lover's embrace. However, with the HYDRA agents gone, play time was over, and they had a mission to do.
She patted him on the back and pulled her mouth away, "They're gone, Dr. Banner. I think it's time to move on." They were on a time schedule, after all, and the longer they were away from the green room, even with kissing, the more suspicion it might raise.