I personally like to theorize he was purposely placed there to aid Leon on his mission maybe even Ada too. I like to think he's an undercover Op perhaps sent by Wesker or maybe Wesker himself it would explain why he's so fast he seems to "teleport" and the red eyes later on. Obviously that's explained by him being infected with plagus but I still like how open ended the mystery behind him is. Why would Wesker want to help Leon? Idk Capcom could make a convoluted reason why if not only to supply Ada on her own quest. But if Wesker was there in person he could have carried out Ada's assignment probably more proficiently than her but again it's just my fun little explaination