[ That it doesn't take her that long to follow up is a good sign for the length of the catacombs. Harold, of course, answers promptly. ]

Make a second mark where you just came out of -- traditionally an X -- and double back. The X means you won't turn down this route again.

When you return to the last intersection, pick an unmarked route if possible, and one with a single mark if not.

[ Yes, this could take a long time. But it is foolproof. ]