I've responded to a few posts asking how to find 'orphan' lines - lines with two dangles not connected to any other lines. See these two posts for clear examples of what users are looking for:
https://community.esri.com/t5/arcgis-pro-questions/how-to-find-orphan-roads-in-network/m-p/1499034#M...
There's a topology rule for dangle lines, but this doesn't address this orphan line issue.
Users need either a new topology rule or maybe a geoprocessing tool to select these orphan lines.
Of note: the solution needs to address an orphan line containing 'pseudo junctions' - see the response/caveat to 'network topology error' link above.