You are the newest adition to the sentry family, for the senario.

You were abandoned by a enemy, maybe blue, maybe something else like the Ghouls, o.W.n, gray mann etc hell you could even have been from another copy cat red engineer. Engineer found you and decided to take you in, so now you've gained a new family, but some may distrust you depending on your origin faction and if you hide info from them(which you can do if you want).