Most likely someone had a cookie set on their computer from when they clicked through on your campaign when it was live. They didn't convert to a lead at the time of the click, but they must have visited the campaign again and did whatever was necessary to convert this time. Your cookie was still set, so you get the lead.
for a lot of my paused fb campaigns, i do get mysterious clicks (about 5-10 daily) and of those, 1 or 2 convert occasionally. really weird i know! also, as steve said, cookie magic