Besides eyeball finds of glass items and arrowheads, sometimes they're accidentally dug up along with the metal item that beeped its presence! Also detector will sound off for any underground bottles with metal caps or closures -- for example, the older canning jars & some wine & beer bottles had a wire bail that secured its lid.
If you want to hunt arrowheads, check the legality first: state law probably varies regarding collecting arrowheads or other Native American artifacts on state public property, federal laws may vary on BLM land and National Forest land, and so forth. Probably most states allow arrowhead collecting on private property. Land owned by Native American tribes cannot be collected on by non-members so far as I know. Many on these forums can give you specifics about arrowhead collecting, though I can't as I'm not collecting them. Andi