Ah, okay, maybe he did capture it after all then. I did notice that it wasn't making any vapor clouds like it seems to when you're unscrewing the lines and a bit escapes. I thought maybe it was just the end of the process and that's why, but nitrogen would totally explain that.

He definitely had a bottle of nitrogen on hand because he used that when pressure testing the new system. And it was just a replacement of the evaporator coil and the condenser, so the line set was reused. Your explanation makes perfect sense, thanks!