Boy, I hope I don't come across as completely weird and inappropriate, here, but my 3-y-o son stopped nursing on his own ~2 and he's still not completely weaned off my breasts, as comfort objects.  This is the 1st time I've breast-fed, but it seems completely normal to me since, as you said, I *encouraged* him to think of them as the source of all comfort, for most of his little life.  I assume that, just as he weaned himself off my milk, he will eventually wean himself off wanting to stick his hand down my shirt! 
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The behavior you've described sounds like him, for the 1st maybe 6 months after he quit nursing. Â Now, a little over a year after he quit, he's chiefly interested in my breasts if he's really upset, hurt, scared or tired. Â If we're in public, I just tell him "That's not polite in public." Â If he's persistent, I might put him down, or step away from him far enough that he can't reach my chest, or - if he really needs a hug - just hug him and kind of move his hands away. Â I try not to be embarrassed about it...and I reassured another embarrassed Mom in church the other day, whose toddler was doing the same thing. Â So I know we're not the only ones!
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But at home? Â I take it as a sign that he needs some cuddling, or is ready to sleep. Â Sometimes, if he's really overtired and crying for no reason, I'll lay down with him and let him put his hand on my breast and he almost always calms down right away and goes to sleep. Â I realize your daughter isn't only doing this when she's tired, but I think if you don't make a big deal out of it, it will progress to that. Â In other words, let her have a little snuggle, when it's appropriate, but make it short by distracting her with something. Â I think that's much less upsetting than trying to introduce the idea that something which was perfectly normal, good and blissful 6 months ago is now kind of shameful. Â If she were 8, it'd be different. Â But if she's 3? Â I don't think there's anything wrong with it.