Leaders, I don’t know how to say this more clearly: Use the cabeceo.
Really, it’s better for everyone involved. I don’t like being shanghaied at milongas. If I am sitting, enjoying the music, maybe chatting, maybe looking for a dance partner, I don’t want you to pop up right in front of me with your hand extended. If I want to dance with you, I will make eye contact. When there is enough light, I can find you from all the way across the room.
Here’s the thing: If a leader pops up unexpectedly and asks me for a dance, I will almost always say no. It gives me no time to think, to consider the music, to size up the leader in question, to think about my mood and preferences … so it’s a no. Your chances are generally better if you hang back. This also avoids you being turned down point blank when there are other circumstances involved. (Recent examples: The first tanda of the night, which I always dance with my favorite leader; a knot in my calf muscle that I was still trying to work out; dripping sweat because of my partner from the previous tanda; and a serious craving for the treats that were about to run out. And I know that you guys rarely believe me when I try to explain these circumstances, but sometimes the excuses really are true.)
This isn’t to say that I never accept verbal invitations. Sometimes I like to take chances, especially with visitors. I often excuse beginners who may not know better. If I have previously expressed interest in dancing with someone, I’ll likely be fine with a verbal invitation (although a cabeceo would probably work just as well in that case). And I do accept them from leaders I am chatting with — but it isn’t a guaranteed yes. I’m just as likely to excuse myself and accept a cabeceo from someone I’ve had my eye on.
So please, use the cabeceo. It’s for your own good.