Consider yourself lucky. We lost our largest 11 year old Koi and three babies from last year from a spawn when we were out of town. Although our family was supposed to be watching for it, they somehow missed the cues (yikes, they are hard to miss!) and consequently didn't do water changes for the ammonia spikes in the water. It was heartbreaking to come home to and a mess as well.
My pond is about 2,000 gal, 5ft by 17ft and about 4 ft deep in the center. We have had multiple spawns in the past 13 years and when they spawn it is very obvious. We have never had submerged plants because the Koi would devour them in minutes (I speak from experience here), and this has never stopped them from spawning. They managed to get on top of the pots of lilies shredding them to the roots with their flailing around. The eggs go everywhere including the rock around the edge of the pond, the skimmer, the sides of the pond as well as the lily pots. Even though they will spent the next few days voraciously consuming the eggs, several somehow remain and three of our five remaining fish are babies hatched in this pond.
For some reason they went about 4 seasons without spawning and then last year commenced to spawning in late April just like they had done in the past. Nothing was different. Go figure.
Foster and Smith has something called montmorillonite clay that comes in a powder form which you can add to your pond which is supposed to add minerals to the water that are good for the Koi.It is the kind of clay that is in the ponds in Japan where the best Koi are raised. It helps water clarity as well. You may want to try that next spring if you really want a spawn.
Good Luck!