Am pretty damn sure I would feel it.. And I would also immediately sense the presence of someone that close to me.
And one more thing that bothers me about this is.. Just how the hell would someone be able to loosen your bra strap as well as unhook it, in the first place? .. Unless of course, that was all you were wearing at the time.. And if so, why were you in that state of undress?
I don't think I wouldn't be able to feel it. Even if my bra suddenly got unhooked, I'd feel it the minute it got hooked off. If it's someone that did it, I'd most likely be pissed. But I'd just ignore it and hook back my bra.