When you deploy a plastic explosive it will always sink half into the ground and it won't show up in the vicinity tab afterwards. After arming it (if the game lets you, but that's another story...) it still doesn't show up in the vicinity tab.
Now if you just put the plastic explosive on the ground without deploying it and then arm it, it won't sink into the ground. Using this method it will also show up in the vicinity tab as well.
I've made a video about it:
This bug has been around for quite a while (if not from the beginning).