Do you have Dropbox installed? In the last weeks there were several problems with its indicator. Have a look on How can I fix this strange icon in wingpanel? and mark this question as duplicate if this solves your problem.
If not the second answer can help you finding out what app is behind this indicator.
Here is the command (thanks to @bitseater):
dbus-send --type=method_call --print-reply --dest=com.canonical.indicator.application /com/canonical/indicator/application/service com.canonical.indicator.application.service.GetApplications | grep "object path"