My old BMW E36 M3 used to drop coolant... about a liter a season... some factors to consider...
1. Evaporation - between the heat of the engine and weather, it happens. After all, what's right under the reservoir? The catalytic convertor.
2. The heater core - if you've been using this lately, and haven't ever topped off the coolant tank, it can hold an easy quart of fluid.
Hope this helps.
