There is such a thing as a "silent" heat where there is little or no bleeding. It's been discussed on the forums before, so you might want to do a search (I don't have my book here for reference or I'd post a bit more about it!)
Boxer bitches also tend to keep themselves very clean, so sometimes it's hard to see the very early signs. What you can do is, after she comes in from pottying, press a white tissue to her vulva and see if there is any colored discharge. Start counting when it is red (sometimes it is a little pink-ish, a watermelon color, but generally day 1 is considered the first day of blood-red). Also, the vulva will continue to enlarge until it is very obviously swollen (like, you can tell from 20 paces that she's in season!). My bitch gets puffy sometimes, and I think she's coming in, and then nothing happens for months. When she finally does come in, I wonder how I could have thought that little bit of puffiness was anything like the swelling she has when she's in