I even did a google search to see if I could find anything & didn't. The only triangle I know of on the android phones is with the memory card you get a triangle with an exclamation point in the middle of the triangle.
This is my guess.
The small triangle above my signal strength bars showed up when I was out of state. Although the manual says that if you are roaming, there should be an "R" above the strength bars, I never saw an "R".
I'm going to assume that the triangle was substituted for the "R" after the manual was written. Now that I'm back home there is no icon above the bars.....all is normal.
If anyone knows any different, please post.