The Volt, BMW i3, and basically any other car with a range extender periodically run the engine in "maintenance mode", because it's good to circulate fluids, burn off some gas, and take some load off the battery for a while.
If you need to store a car for more than a month, you should top off and add a bottle of fuel stabilizer before driving home.
It appears that the Volt will detect the gasoline going bad (or at least, puts it on a timer and then assumes), and forcibly burn it off roughly once a year. Before doing so, it encourages you to use a bit of gas.
To be honest: this is the first link from a search engine. So its not an issue I've looked into very strongly. Nonetheless, it seems like GM has already figured out a procedure for "stale gas" issues.
https://forum.quartertothree.com/t/volt-gas-goes-stale/69063
So it seems to be a timer of some sorts, roughly on the order of ~once a year or so.
EDIT: I found a video on youtube showing the process: https://www.youtube.com/watch?v=mEe4Rkg4GfA
The author of the video notes that you can drive the Volt just fine. He just had a camera handy and stay'd in his garage in this case however.
* Keep the tank full of gas: air space is prone to condensation
* Avoid ethanol-added gasoline, it's more hypro-philic
* Add a fuel stabilizer (like 1 fl oz per 5 gallon)
* live in California, it's really dry.
These are adapted from techniques for winter storage of cars.
Engine Maintenance mode: haven't used the gas engine in some period (month?) so it uses the engine for a small period of time (less than my 12 mile commute) so get the engine oil moving.
Fuel maintenance mode: doesn't let average age of gas be over a year. i.e. I filled my tank August 30, 2017 from basically empty. On Aug 29, 2018 it went into fuel maintenance mode and even though I had a full battery, it would only use gas. I was probably around 1 gallon left, I filled it up, so expect around june 2019, it will go into fuel maintenance mode again unless I finish off the tank and fill it up again.