First of all, I don't recommend the ordinary person enter or explore ancient tombs.....my feeling about it, is they are for the archaeologist to study. I'm not willing to except any treasure found by such activities either. No harm in studying sites, but there are antiquity laws to consider.
Anyway, I've had treasure hunters checking out dowsed maps find entrances to them, including Roman tombs. Most of these were marked by red line rectangle boxes with solid white circles in or near them. The map with the red outline circles was only for a shallow dowse of the area 2ft or less deep. To show you the difference, here is from my file of definitions for marking dowsing hits.
"Red circle is to mark areas, of loose or surface coins/treasure and artifacts of value, down to 1-2 feet deep.
Red line box (or solid filled circles) for very deep coins/treasure or artifacts of value."
White marked spots for my dowsing results on maps, can be a number of things, especially if not close to red marked spots. Sometimes only a simple void or shallow natural cave. From my file of definitions for white.
"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.)."