1999 4CYL Toyota Camry

So the car is currently in the shop but they cannot locate were the leak is coming from and I am starting to get very frustrated considering Monday will be two weeks it has been in the shop. They have replaced the timing belt because of the oil leakage also they have replaced the seal and spark plugs which I know have nothing to do with the oil leak but apparently they did that as well. So the bill is up to 500.00 and I spoke with him on the phone today and he said that hes going to take everything apart again to see if he can find it. They said on the phone they think its the "Cam" but don't know. What I do know is that we didn't know it was leaking in the first place because there was no oil on the ground that was visible. If the guy who went to change the oil hadn't told us there was a leak we would have never known it was leaking because it runs normal. I don't know much about cars so I can't really tell you much other than what the mechanic has told me. So any advice would be appreciated because I don't have the money to keep shelling out to this guessing game.