What would cause an Airport Express 2 to disconnect from home network overnight?

What would cause this?

Any number of possibilities, some of which might be.......

• A momentary power AC power "blip" that you might have not noticed at the time
• WiFi interference from another router near your home
• Interference from other electronics like a microwave oven
• Momentary interference from a cell network, including new 5G networks that are coming online
• The internal power supply inside the AirPort Express that might have encountered a "blip". Remember....your AirPort Express is at least 5+ years old, and as much as 9+ depending on when it was placed into service. On average, the useful life of an AirPort Express is about 5-6 years
• Gremlins

If you continue to encounter disconnections and/or the disconnections increase in frequency, the most likely scenario is that the AirPort Express is starting to show its age.

Can I deduce its age from the serial number or other data in any way?

Possibly, but not reliably. The forum moderators tend to delete links that we post for things like this. If the link is still up.......Try.

https://www.imeipro.info/apple-serial-check.html

5-6 years of useful life is an "average". Some go longer, 8-10 years or more and some......like my last 2 AirPort Expresses.....die young at less than 4.

To rule out WiFi interference or other airborne interference possibilities, you will need to reset and reconfigure the AirPort Express to connect to the AirPort Extreme using a permanent wired Ethernet cable connection. If the Express still tends to disappear, then you will know that it is already on a downward path.

Your experience is not unusual. There are some things you can do to help the Express NOT disappear.

We do need to know the whole network layout.

What make and model is your main modem / router? That could be one or two separate items.

How is the Express connected to the network? Note. Ethernet is always superior and should be used if possible.

What device are you using the airport utility on? What OS is it running?

What device is used for airplay to the express? How is it connected to the network?

Express or any other Apple router used as AP can easily disappear from network when teamed with a router that does not understand Bonjour (mDNS) very well. It is time for Apple to move fully to SMB networking since it is already their preferred network protocol.