Was it a "get out of my way" honk or a "I just want you to know I'm behind you" honk? My mom, bless her heart, has some fear that bicyclists don't hear/see her and that they are going to just turn in front of her or something. So, if she is leaving her lane to pass one, she gives a little "toot" to give them a heads up. I've explained why this is a bad idea and unnecessary but she feels she needs to acknowledge the bicyclist saw her before she feels it is safe to pass. SMH.