I did not look to the hydrant Wiki, but know that ref is a common key used for several objects. It was somehow a misunderstanding, I thought Vespucci wants me to enter a tag “reference” (written out in full) instead “ref”.
Nonetheless, it just bothers me that Vespucci repeatedly prompts me to add a value to theref tag and it becomes annoying when there is no actual value on-site to enter. Especially since the Wiki correctly states: ref=* ... if applicable. Therefore, what I would like is for Vespucci to stop being annoying.
You can either simply turn the check off Preferences - Vespucci overall, or create a version of the hydrant preset without a ref tag (and some other variants like proposing a regional variant of the preset).
The cog icon is the current icon for the preferences. Creating your own preset is not difficult but on the other hand it does need some relevant experience, I would suggest simply disabling the “missing tags” validation as a quick way of suppressing the warning.
Objects that fail validation are highlighted (colour depends on which validation, in this case the preset driven “missing tag” warning) and potentially generate a notification … the original highlighting goes back to ~2010, notifications back to 0.9.7, a decade ago.
Then maybe the missing of a ref on fire hydrants should not fail validation? Where does Vespucci source the information from what to mark as a warning?
You can change to this version by following the instructions here Videos & Screenshots - Vespucci or simply wait for the ~mid-month maintenance update of the app.