Tenant keeps complaining when nothing is wrong. What should i do?

[Jacob C Olinick]

One of my tenants has been constantly complaining  bout the water bill. She claims something is leaking in the walls but there are no signs of water damage anywhere. I have gone out to the property several times and the outside meter is telling me no water is running or leaking. I have been managing this property for many years and never had a tenant complain about the water bill before. She isn't a bad tenant, so I would like to renew her lease because she pays on-time, but I can't keep going out to property every week to tell her nothing is wrong. What should I do?