Lukas was spying for Deku. People had been suspecting for a while that Katsuki might be working with the LOV in secret, and Lukas was REALLY starved for cash. Plus he liked Deku! He was always nice to him-so when he asked for a favor, Lu was more than willing to help.
It was one night..9 pm. Katsuki was..."Patrolling." The thing is- he wasn't walking the usual route that he takes. He was heading to a warehouse, and even better...Lukas knew his route that he took for patrolling.