Well, it’s pretty much all been said: Men/women, different, women/women, different, sensitive/not sensitve boobage, etc.
I think, though, as a general rule, this will always be true: A straight man will notice boob contact against his own person in the following manner: first, fast, and way out of proportion. Does it ever happen that a woman does it on purpose? Yes, certainly, and probably all the time. I do not claim to know what the purpose of their doing it would be, that would vary. If it were myself? Probably flirting, and it would probably happen as often as possible. I don’t do this, however, and so it is moot. I do stuff like that to my husband, and there’s often no reason to be coy unless we’re in public and I want to give him a secret smile. Does it ever happen unintentionally to men I work with? Oh, hell yes, at least with me, and I’m certain I’m not the only one. If I notice I’m doing it in a situation like the OP, I might say, “Oh, excuse me,” and lean back, with a minor amount of sheepishness lingering. Some women may not do this. If they notice it, they may be too embarrassed to say anything, and simply ride it out. Others may simply not care, thinking, shit, it’s just a boob. We’re a little desensitised, sometimes, you see; after lugging them around for most of our lives, sometimes we think most people are both aware we’ve got them and that sometimes we bump them into something, and it could just as easily have been an elbow or a shoulder.
Any time I’ve done it accidentally, the man it happened to has let me know about it. I work among some pretty rough characters, however, and they have no qualms with treating me like one of the boys - a boy with boobs, that is. A memorable anecdotal incident:
I had to lean past one of my co-workers to grab a lid opener for a can of paint on the table we were both working at. I didn’t notice that when I leaned across him, my breasts has brushed against his back. You see, I wear a sturdy bra, then my work shirt, and on top of all that, my stiff work apron when I’m dealing directly with paint. Didn’t feel anything, got my opener, resumed my orginal position beside male co-worker.
Co-worker, staring and looking wounded*: “You just assaulted me with your boobs.”
Me: “Oh, shit, sorry, didn’t notice.” Pick up my can and leave to speak to a customer.
I return to the back warehouse. Co-worker is looking hurt, speaking to manager**, pointing at me.
Co-worker: “She assaulted me with those things!”
Manager, shaking his head, trying not laugh: “Yeah, well, we allow women in the workplace now, it’s bound to happen sometimes.”
Co-worker, brightening: “I’m calling her husband!” Gives me mischievous look as I shake my head, smirking.
Me: “Go right ahead.”
Co-worker, calling other store: “Hey, [my husband’s name]! Your wife just assaulted me with her boobs.”
My husband: “Oh yeah? How much did she charge?***”
As you can see, jokingly or not, this was a far bigger deal to my male co-worker than it was to me. 
I won’t say it wasn’t anything to me; I obviously am amused enough by the story to remember it, retell it, and sit back and chuckle that these babies cause some ruckus sometimes. But it certainly didn’t mean anything.
Just keep yourself safe, as others have mentioned: there’s far more at stake here for you to lose than for her. It’s unfortunate and unfair, but take care of yourself. Better to get to know who she is as a person; let her tell you she’s interested with her eyes, gestures, and frequent conversations rather than occasional boob brushes. You’ll be a hell of a lot safer that way.
-
- he’s not serious.
** - neither is he.
*** - he is****.
**** - I’m not.
