I can't remember the exact location of the square hole but this leads to the valley of the engine "upper" drain area .
Here's a link to a post that shows the exact location of the round and square holes, and also the path fluids take to get to these holes:
ST1300 - Water Pump Weep Holes.
Be aware that when you take the motorcycle out of storage in the springtime, and give it that "first really good wash and super-detailed cleanup", you will probably spray some water from the hose into the area between the 'V' of the engine cylinders. For sure this will happen if you have lifted the upper fuel tank up. When this nice, clean, innocent wash water runs down into the V, a little bit of water will remain down there, and it will dissolve any evaporated coolant deposits that might be present around hose clamps. Later, when you roll the motorcycle into the garage, you might find a green stain on the floor when this water drains out the square hole. This does not mean you have a leak.
The rule of thumb for diagnosing and evaluating leaks from the square hole is to never assume that a
single instance of greenish fluid leaking out the square hole means you have a coolant leak. Wait a few days until the motorcycle has gone through a few operational cycles (from cold engine to fully warmed up engine, plus a little bit of riding around). If you don't see a second instance of green fluid leakage after a few operational cycles have passed, all that happened the first time was that you washed out years of accumulated crud from the V in the middle of the engine.
Michael
