Well if it is the REAR cam seal then I would think that it wouldn't leak on the timing belt but would rather make a fat mess down the back of the bell housing. You do not do too many of those seals but the ones that you do are a straight forward slam dunk affair. I have never had one come back. This leads me to wonder if you aren't leaking through the cam carrier right by the rear cam seal. The carrier is sealed with a liquid sealant that is akin to loctite. Could be that when they took the head off it did not seal back there in one tiny spot and now it is leaking and it looks like the rear seal and it is not. Let's face it, if your guy is using Volvo seals and the Volvo tool to put them in with then the odds of him getting 3 bad seals are pretty slim. This means that your leak is somewhere else, albiet right in the immediate vicinity of that seal. Has he tried cleaning it all up, putting some dye in it, running it on the hoist until it starts to leak, and then inspect it with the UV lamp and glasses? That is where I would start if it was in my stall.