It's supposed to happen by entering the Bus Stop from the Farm during any morning or afternoon in Winter. Since you've been Foraging, you certainly have done that at some point during the previous 4 Winters. I suppose if you missed it or didn't notice where the Shadow Guy (not necessarily Krobus, maybe his sister?) went during one of those times, the bush where you're supposed to find him and the Magnifying Glass might still be able to be triggered by nudging that bush a few times.
On my first playthrough I missed the hints about where to look for him and that may have been my first foray into the SDV Wiki to see where I should search (the in-game hints are obvious but I have a very thick head and had trouble early on). I don't think I found him on the day he appeared (when the cutscene triggered) and searched later to find him. Hopefully you can, too.