could be a leak in the sprinkler system

shut down the sprinkler system, and see if the meter still runs.

If it is the sprinkler system, then need to get a sprinkler guy to do the repair.

It could cost you up to several hundered dollors, depending on how difficult it turns out to locate and repair the problem.  if it was me I would be doing it myself.  I installed the whole sprinkler system in my yard.