A hidden entrance (behind a bookcase, beneath a rug, a hatch in the ground, in the back of a pantry or closet, past the back panel of a cupboard or wardrobe, a wall seam hidden by a wallpaper pattern)
Rough walls (made of brick, stone, dirt, mud, or wooden boards)
Stairs leading below the floor level
An uneven floor covered in dirt, dust, or spackle
A narrow path that has sharp corners and snakes between rooms and hallways or is deliberately straight and purposeful
Walls that are coated with dust and dirt
A storage area for emergency supplies (food, water, blankets, candles, flashlights, extra clothing)
A door leading to a hidden room that few know exist
Alcoves and passageways opening off the main path
Tree roots poking through the walls (if it's below ground)
Water dripping from the tunnel roof and puddling on the floor
A flashlight beam trained on a floor littered with fallen bricks or stones
Old pipes and dangling wires overhead
Support beams at intervals
Spiders and spiderwebs
Rats, mice, and bats
Evidence of squatters or former residents (food wrappers, crumpled cups, moldy sheets and blankets, cardboard pieces laid over the dirt floor)
Darkness
A small lit radius from one's candle or flashlight
Dust motes floating in the air