The pin you refer to is actually the pin fitted in the sear arm and it is exactly that, a tapered pin, not a screw with a thread into the lockplate. The sear pivots on this pin. The pin itself is not integral to the sear and was tapped into the sear from the reverse side after the sear was made.
In your case, the pin had backed out of the sear. If you remove the lock you will see the other end of this pin protruding out of the bridle. Gently and accurately tap it back in (as to not dent, damage or pinch the bridle and sear) or dismantle it and see why the pin has backed-out/loose.
Nothing serious to worry about, it should not travel back anymore as it will stop hard again the interior wood face that is recessed for the bridle.
Cheers
J-PD
