I couldn’t find any possible source; but a few hours later, when I started to feel lightheaded from the smell, I did an online search and then reported a possible gas leak.
Within about an hour, this nice guy, Tony, from PG&E appeared at our door. He used a special tool to search all over our house and near the gas meter in our yard. Thankfully, there was no gas leak! We still have no idea what caused the funky smell, but we’ll sleep better knowing it isn’t gas. We sent Tony home with much thanks and freshly picked lemons from our little tree!