On avait,alors,ouvert les rideaux pour découvrir, éberlués...

Yes, on stands for "we".

When 'on' can be replaced by the specific subject pronoun 'nous', adjectives agree with number and are therefore plural (only the past participle/adjective, not the auxiliary verb conjugation).  

For what it is worth, I have come across tutorials by native speakers who openly admitted they were unaware of this agreement rule until preparing specifically to cover 'on' in a grammar lesson (probably learned at school and forgotten though) ! It is, of course, only really relevant when writing.

Here is a lesson link : On can mean either we/one/people (French Subject Pronouns)

Having read the explanation by Laura Lawless with the quote from Le Bon Usage, and also knowing that not all native French speakers know or follow this ‘rule’ of agreement either, I think it is pretty safe to ignore it in most situations.

It is unimportant in speech and irrelevant in text/social media forums. It may be worthwhile remembering if doing very high level formal French writing or tests. In the latter situation, erring to the use of the ‘optional’ agreement may be easier than arguing the fine points of the explanation in Le Bon Usage !

Thanks guys!