How did he do this?

Lert, on Jun 27 2014 - 16:32, said:   Abbey. Central map area. My toonmate and me are in T-44's. We took the central abbey at map start, spent some time holding down the ramp, when an IS-3 appears behind us and starts shooting us in the engine. No way he could've gotten there before us without being spotted by us, so he must've gotten there after us.   How did the IS-3 get there?   Don't get me wrong, I am not claiming hack and not accusing him of cheating. I just want to know how he got there.