2-Man crew loophole?

[pingswinger]

With R1 and R3, B1 hits a foul fly on firstbase side off into foul ball territory. With a lot of foul territory, the flyball carries first baseman way over by fence. My question: On a tag situation, I found myself on 3B line extended way over by dugout checking the catch/no catch on the foul ball. It then dawned on me that the runner could have advanced at third without me checking tag. Is this my responbility or BU responsibility.