Hey enable_15,

This is an object which was surveyed (not added from aerial imagery) by a user who likes to add additional detail. This might be correct (solely inaccurate) data, though it is not always flawless (first part can be checked by looking at object history, second part you would need to know the mappers of course, third part some tagging experience is useful)
It is likely they added 'unknown' because it is not signed on this specific object (numerous old sewer vents have their manufacturer signed, modern ones do less/not)
It might be better replaced with manufacturer:signed=no .
Understandably, contributors do not always take a (permanent, compatibly licensed) picture, so it is okay if this is hard to verify for you.
You can always ask the user in the changeset where they added the tag if you are unsure.

I would suggest starting with simple mapping for your first changes, not jumping into challenges and automated edits straight away.
Generally, you should start with small changes and keep them local
https://wiki.openstreetmap.org/wiki/Changeset#Geographical_size_of_changesets
(here: size is proverbially speaking, this specific changeset's size (single object) is fine.)
Otherwise, make sure you check each object manually and properly research, document and communicate your changes.

See also
- https://wiki.openstreetmap.org/wiki/Good_practice#Don't_remove_tags_that_you_don't_understand
- https://wiki.openstreetmap.org/wiki/Automated_Edits_code_of_conduct
(includes 'semi-'automated changes)

If you need help mapping, you can contact your (local) community at
https://openstreetmap.community/
or start a thread at
https://openstreetmap.community/

Regards,

Daniel