Nearly every night you know! At ungodly hours. Yesterday it was at 23:45, the previous night maybe around 4 am.
It is some kind of a siren and loud enough for folks living around Bahnhof Affoltern to be woken up to shut the windows
Does anyone know what it is?

ze germans are coming!

Probably a incoming train warning for workers on the railway-line.

You have you windows open at the moment? Hardcore!

Not sure about siren, we have a whistle that goes on past midnight doing our heads in at Albisreden.

There is must be a secret Swiss system going on

Yes that is almost certainly the source.

I think they are called church bells.

Don't get me started about the noise from the church bells. Some days I think they are never going to stop - bong bong all hours godsends

yes i prefer the oriental chant of minarets

I have a friend in Wettingen who, when they came round to collect the Church Tax, offered to pay double if they would "shut the f'ng bells up after 11pm at night".

It's the foreigners warning siren.
Any foreigners caught out of doors at that time have committed a crime and will therefore be deported.
Remember people, you are foreigners and should stay hidden.

LMAO Its too expensive to be out

If you see someone standing on the street corner in the early hours of the morning with no-one else around and no traffic

It is either, a foreigner up to no good, or a local waiting for the green man ;-)

Oncoming trains at 4 am? But it only started recently - wasnt there until about 3 weeks ago. Going by the Swiss timeliness, one would expect the siren to be at the exact second each night - but nah!
Anyways, I hope 'they' (?) let us sleep peacefully soon..