Some of my cats love to go outside, especially in the summer and early fall. I guess so many intriquing smells and the warm concrete attracts them. She may get out of this come winter....but she may now. You may want to invest in a harness and take her outside supervised for a little bit each day so she can get her "fix". Mine just run free, but we live at the end of a dirt road so traffic (my main fear) really isn't an issue.
One question though...she is spayed, right? If not, she could be putting her scent out there to attract a mate. If she is, then I'm guessing she's just enjoying her time outside.