So... I'm an idiot, and I left the propane tank connected to my grill. With the tank valve open.
Both knobs on the grill itself are set to "off" but I already wheeled the grill back inside into a semi-screened-in porch (it's screened in, but there are plastic windows that cover them, but do not seal). It was 1/3 or 1/4 full when I left, and it's a 2 hour drive away, and someone's headed out there on Wednesday. Have I seriously endangered my house and should I drive back out there sooner than Wednesday?