I fly with my D10, and LD01, and LD20, and streamlight nano, and zebra head lamp all the time. I also have a camera case with 8 extra AA batteries with me and not one second look.
My experience has been that an aluminum 1xAAA flashlight does not set off the magnetometer, even at airports where they're set to be very sensitive. I'd be very surprised if a 1xAA or 1x123 size did either.
In a carry-on bag, no flashlight has ever caused me problems. As gbelleh says, I'd stay away from scary-looking crenelations. I'd also stay away from anything that looks like it could be a bludgeon.
I cannot see any reason why they could complain about your carrying of a small torch on a plane.
Worst case would be they take a look at the light at security, turn it on and check it is a light and not a small bomb or something, but it has no sharp edges or anything...