Fork of "https://mods.factorio.com/mod/Orphan%20Finder" v1.1.3 by GotLag, updated for Factorio v2.0, until such time as GotLag updates their own version.
All credit to GotLag - all I've done is made some minor changes to get the existing code working under Factorio Version 2.

Original Mod Description:

This mod adds a hotkey (Shift+O by default) to highlight any "orphan" underground pipes or belts near the player, that is to say, any underground pipe or belt which doesn't have another end connected.

Press the hotkey again to hide any existing markers. Individual markers will also be removed on mining the highlighted pipe/belt, or rotating or building other pipes/belts to create a matched pair.

Loosely inspired by matjojo's Underground Belt Spotter for Factorio 0.12, but with entirely new control logic and support for pipes as well as belts.

Compatible with modded underground belts and pipes.