You could have a water leak internally. For instance, a toilet that is slowly leaking and refilling. I’ve pumped many septic tanks and had water flowing with nobody home. Quick search and “Oh, would you look at that. The toilet is leaking.” Neve would hav known otherwise.
I agree with the previous posts. Check the meter current against old bills, cut off records from the company, etc.
Also, seems like really bad luck that this leak started right when the tenant left. I’d really want to see the current meter reading and trace it back through all the previous bills to see that there isn’t a clerical issue.
Lastly I have a friend this happened to on her own home. $50 month for years. Then suddenly a $1000 bill shows up. Utility didn’t care. Pay it or we play hardball. No negotiation. No forgiveness. Maybe they will listen, but unless you can document something wrong, I wouldn’t get my hopes up. In other words be prepared to pay the bill. They will structure the payment over time so cash flow isn’t hurt.