Unless you unplug everything when not in use, you already have a dozen fire hazards at home. A PC that's powered off still delivers power to a bunch of components like the Intel ME, or nic for WOL; your TV, dish washer, washing machine, fridge, Alexa, smart hub, router, PlayStation 4 have some soc running 24/7. A pi is probably very safe, I've never heard of one going up in flames just like that. It's much more likely the USB charger it's running off will blow up of it's a cheap one from Walmart.