Could you also say "Nous sommes passés la poste." (without the preposition 'devant'.

No, I don't think the sentence "Nous sommes passés la poste" is proper French. There does seem to be the requirement of a preposition of some kind, although it doesn't have to be "devant". You could also say, "Nous sommes passés par la poste", for example.

But other meanings of "être passé" don't require a preposition. For example:

Le carburant est passé de l'état liquide à l'état gazeux. -- The fuel changed from liquid state to gaseous stat.
Le temps est passé. -- Time's up.

-- Chris (not a native speaker).

Thanks Chris, I suspected that would be the answer but I wasn't sure.

Hi Stewart,

You can say,

Nous sommes passés à la poste - We went to the Post Office

Nous sommes passés devant la poste - We went past the Post Office

Hope this helps!