The sign is located behind/nearby.
Top line should be diameter in mm.
Bottom is distance to sign in meters.
https://wiki.openstreetmap.org/wiki/Tag:emergency%3Dfire_hydrant#Optional_tags

If StreetComple quest is unclear, feel free to open an issue here
(top right click "New issue")
https://github.com/streetcomplete/StreetComplete/issues

Resolved by
https://www.openstreetmap.org/changeset/158503419