If no visible signs of a crack, it may be the sealing ring /gasket in between the two parts, the bolts holding it together are used to clamp it and on assemble when loose the two parts can move to align oil pipes and exhaust etc. and then tightened up to be held secure, usually for life of turbo.
I doubt if the gasket ring would be available on it's own and due to to the trouble taking it off, personally I would just keep an eye on it as long as the bolts are secure, as long term it will probably involve replacing the turbo, in which case get a good quality one as there is a lot of cheap rubbish about.