While sparring with a friend I tried to mark on the map where I had laid my caltrops and as I began to draw it on the map suddenly everything went white, and only he and I were able to see this place/the markings. Our DM and a Barbarian could not see anything we were talking about. Apparently however, our DM was checking on some last minute stuff on a different map for our game on Sunday and she had found our markings! Not sure what would cause this but it's a good thing the map didn't load on our end or it could've been a significant spoiler.