Slitherine

I'm playing as Syracuse. After recruiting a Mercenary Hoplite unit manpower upkeep drops by -2 but on the unit card it says there shouldn't be any manpower upkeep cost.

By default, each faction gets a flat upkeep reduction for 10 of their units. Most likely, your Mercenary Hoplite replaced an unit that costs less money, but has a manpower cost among that 10, causing your manpower expenditure to increase.

My bad then. Thanks for the explanation.

That's not obvious to understand why and Soar explanation is most probably the right one indeed.