The window of Grantaire's room is just able to see the greenhouse from the right angle, and although he's not actually looking out it at that angle, he's painting early one evening and sees a burst of light out of the corner of his eye and goes to the window to look. It's rather clearly coming from the greenhouse and although his first instinct is much like it was with the zombies, that being to ignore it and let whoever feels like dealing with it do so, he can't help a bit of curiosity in this case. It's even enough to make him brave the cold.

So a brief time later, he's cautiously stepping into the greenhouse with a coat on, peering around to look for the source of the light.