Peter, the ‘s’ is only relevant when using ‘on’ in place of ‘ nous ‘. 

When ‘on’ is used as an indefinite pronoun, it is invariably masculine singular grammatically, both for verb conjugation and adjectival agreement. 

However, when ‘on’ replaces the definite pronoun ‘nous’, the usual rules of adjectival agreement occur, even though the verb is still conjugated in the 3 rd person singular. It is really only relevant in writing, and I have seen, on more than one occasion, admissions by French native speakers that they only became aware of the rule themselves when preparing a grammar session for their own online courses ! 

The link below should lead to a recent discussion on this topic as well:

 https://progress.lawlessfrench.com/questions/view/follow-on-qu-on-etait-eberlues