1998 Pontiac Bonneville Oil Leak

Took my car in for an oil change. Mechanic says there is a large oil leak, but that he can't find it anywhere. He wants to clean out my system and insert a dye to find out where the oil leak is. First of all, when i park my car and come back to it i don't see any oil spillage. If there was a large leak, wouldn't i see some spill under my car? Secondly, if there is an oil leak, and it will be an expensive fix, am I okay with just adding oil when it gets low, or am i in danger driving the car with the leak? I have a 98 pontiac bonneville with 200,000 miles.

Thanks.