2009 Journey has coolant on floor of garage on passenger side. Car doesn’t overheat but had to add coolant to the reservoir. Mechanic can’t find leak.
If its really coolant, then heater core is probably leaking. Surprised your mechanic can't find that.
If its just water (even dirty water), there should be a drain (if you've ever seen a car drip water on a hot day with the AC on) have your mechanic verify the drain is clear.
Both are typically on passenger side of car.