To be honest, I thought your email indicating "<" (left arrow) was a typo and that you had meant to write ">" (right arrow). Then I checked your web site and I see that you have indeed used "<" on the right hand side of the menu item to indicate expansion which I don't believe I've seen used before...
In my opinion, the common convention is a ">" (right arrow/triangle shape) on the left of a menu item indicates available expansion, and it changes to "down arrow" when expanded.
Plus/minus is the same; ie. should be to the left of menu items with "+" (plus) indicating available expansion and changing to "-" when expanded. Plus/minus was much more common back when we all only had text based terminals.
A "down arrow" to the right of text (or an icon) indicates a "drop down", or "pop-up menu" is available.
The fundamental difference being, a "expansion" menu happens in-place (pushing other items out of the way) where-as a "pop-up" or "drop-down" menu appears over-top of the existing text.
That being said, absolutely anything can become a "standard"; it only needs appear in something popular enough that people get used to it. Arrows probably started replacing +/- because it got used in something popular (Outlook?). I'd be interested to know the history of these "conventions"....
In the "real world" there are actually written standards for these things (called human factors). That's why when you see a door knob, you "intuitively" know to twist it, not just pull or push. That being said, the standards are frequently ignored. If you've ever walked up to a door and tried "pulling" when it's actually a "push", it's likely because someone ignored the standard and used a shape that implies "push".
Sorry for droning on about this but I find Human Factors a fascinating and very overlooked area. A high proportion of "mistakes" could be avoided with better design. When they are ignored, tragedy can result ("medical error", "pilot error", Chernobyl, etc.).
P.S. I had to google what a "hamburger menu icon" was....
John