Recently, the following indicator appeared in the top bar (see screenshot below, I'm running Freya). I can't get any option or other information by clicking on it, the menu bubble is empty. Thanks for any help.
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"