@Sarah977I run septic too and a guest could accidentally turn it off with a switch that looks like a power socket or light switch ... guests don’t need to go to the electricity board to do it, hence my comment.
When people are new to a house they don’t know what switch turns what light on or off which can lead to accidentally turning power off.
If it was turned off in the electricity board that is a different matter ... though that should be locked for guest safety.
The other consideration is that if it was off in the electricity board is that it may have tripped or another guest turned it off ... I know our septic doesn’t usually fill up and cause plumbing issues for weeks or months.
I personally dont think a guest would go to the electricity box, deliberately switch off a “septic” switch and expect plumbing issues, just to get out of paying for a few nights.
My advice was given purely in good will and what I would do based on my own circumstance, property and experience. 😊