Just because she walked past a bong, it doesn't mean she actually saw it, or took it in, or even knew what it was. Most people who aren't exposed to MJ culture don't know 100% what a bong looks like. You might have collected a fancy water pipe from when you visited Morocco, which you have as a centre piece. Or just had a nice 70's style vase. Or it was part of a cat toy you were building.

If she brings it up at work, you can easily deny it; just say it was something else and really stick to your story, but act like it's not a big deal- because after all if it was just a vase, it wouldn't be.

If she's adamant it's definitely a bong, you can say 'yeah it's SO's, he has back pain and uses it for that, I hate him leaving that damn thing in the living room', or 'I hate the smell but i can't get him to quit' and brush it off as not being yours, whilst simultaneously cementing your position as not liking MJ.

And the end of the day, work can't come round to your house to prove it's yours or whatever, the most they can do is spring a random drug test on you, so now might be the time to find a clean pee contact you can rely on at work, or order yourself one of those drinks which will let you pass the test.