That looks similar to the volume indicator in my cinnamon but it's not actually for the volume by the looks of it, the size shape and position's the same but I don't know what you'd call the one on your desktop. I didn't know there were any other notifiers like this for anything else, be interesting to find out what it is . . . . . sorry i can't help though !
Lenovo ThinkPad T61 LM14 & 15 (different HDDs) Intel T7500/2.2GHz/Cinammon 1.6/1.8 Intel GM965. Toshiba Satellite M70: Dual boot Zorin OS & Elementary.Desktop:LM13/15 Geforce 9500 GT, Athlon 7750 Dual-Core 2700mhz,3gb RAM.Linux Registered #562462