Yes, same artist and likely on different visits. The thing with these places is there are so vast, you can hide, draw for hours.. and security will never find you as they are not allowed to go inside the buildings.
Derelict buildings lie those which will either be demolished or extensively renovated seem like perfectly safe places to practice. I don't see any real harm done. Of course, injury risks and liability laws certainly can change how that is perceived, but as tagging goes, it's pretty innocuous.
I saw one just once, long ago in Sheffield at an explore. He was friendly, and even gave me his card .. (yes, he had one!)..