In over 20 years I have been approached three times. Twice was a tap on the shoulder and a nearly whispered "your printing" and then they both gave a grin and patted their own EDC to show me they carried as well. Only other time was by this woman who approached me and asked in a real snotty voice, loud enough so everyone around us could hear, if I was carrying a gun? I gave her a puzzled look and asked her what made her think that and she said that "obvious bump in your shirt" (IWB appendix carry). I looked her in the eye and answered in an annoyed voice, "Oh, you mean my colostomy bag? She turned beet red and could not say she was sorry enough times. I had a hard time keeping a straight face with all the snickering going on around me.
IMHO I think that carriers notice it more than John Q Public as I will notice little things that tell me so and so is carrying which if I didn't carry myself would not have even raised an eyebrow.