Sounds like this crossing was properly marked, and simply missed. I will state, most of us who ride enough have made this mistake a time or two, and came out without incident.
I was riding the past two days near Muni. Two different times I saw signs were missing. I went back both times to looks. One, the sign was turned 90 degrees from the groomer. What it appeared was the drifting was so large, that when the blade was down moving the drift, it was so heavy and large it twisted the sign. I did my best to straighten it, but was still not good. The second sign I could not find due to the deep bank.
I am not saying with this situation deep banks were the problem, but we all know with this wonderful record snowfall we had during Feb. it has also created some dangerous conditions. The photo below, while not is an issue everywhere, shows the issue with banks. If you are riding right now, you know some signs will be covered. And, what I think is almost worse, the trails are narrower and you can't see oncoming traffic. Almost every corner is a blind corner. I did have a group running hot around a 90, well over their 1/2 of the trail. I had no where to go. Couldn't even jump off the sled to my right if I needed to because the bank was so high I would have made it up high enough. We didn't hit, but it was close.
Be careful out there.