Which app does this menu bar icon belong to

I don't recognize the icon, but I may have a way to find out which app it belongs to.

Bartender is an app that organises the menu bar apps, but can also show which apps are already there. It's a paid app, but you can install the trial, find the mystery app, then uninstall Bartender.

A screenshot from an installation of mine.

Tip: If you just want to remove the icon you may be able to ⌘-drag it away from the menubar, but for that to work the app needs to be an Apple built-in app or use the same menu bar hooks.

Use the Accessibility Inspector, accessible from Xcode → Open Developer Tool → Accessibility Inspector.

Click "Show" on the "Hierarchy" sub-section (which will show you the Application Name).

Click on the "Target Icon" at the top of the Inspector (Labeled: Start inspection follows point).

Hover over the item and the pane may show you enough information for you to determine what it is. For example, hovering the Dropbox menu bar item shows "Dropbox <version>" under AXHelp, and hovering Little Snitch shows "Little Snitch Agent" under AXTitle.