I used a rectangle which indicates deeper on red marked targets. This is where I got a signal, but no certain hit on entrance. Usally, I will mark an opening in white. You can have mine workings that got covered up. Then if it was a cave, might be another location. Best thing is to check it out, if there is an entrance, most likely concealed. Here is guidelines from my file.
"Red line box (or solid filled circles) for very deep coins/treasure or artifacts of value.
White in general is a color indicator, I use for natural caves or man made tunnel entrances and mine shafts, other similar type excavations (including old wells, tombs, etc.). "