I believe that's an artifact, not shadow of any sort. I run into this many times when using similar fake.
When you separate reflection and light in Corona it often wreaks havoc in some situations, particularly on such glossy material like that ocean pier has.
Here are my two solutions:
1) Use Reflection override in Glass material (not global reflection override, just the Glass). You can CoronaMix the Environment + Your Plane in some creative way and plug that it.
2) If it's not animation, nothing bests post-production on masked windows..