About 13 years ago, they wanted my husband and son to remove their tzitzis and put them through the scanner with their jackets going through security in ORD. My DH refused. I suggested tucking them under their shirts, but the guard refused, so they ended up getting patted down. This despite my DH immediately lifting them to show that it is simply a piece of cloth.
I have had my head patted down in many domestic and foreign airports because of my tichel, but they never asked me to remove it. I did have once where the security person asked me to confirm that I wear it for religious reasons, glaring at a woman nearby while she asked. I guess that the other woman had said something about my tichel.