I shut off main water valve, but water meter shows water usage.

[Steven Tu]

Before I was out of town for 1+ month in January-March (snow months), I shut off the main valve. However, the subsequent city water bill still showed a non-trivial water usage (6 CCF). I checked and didn't find any in-house water leakage. Is the water in the sun pump included in the water bill, since my sun pump is operating frequently?

Thanks.

Steven