2001 Buick Regal LS - smells like fresh gas

3.8 engine, over 100K miles. I have no idea where the smell is coming from. I have no obvious leaks, no obvious reasons that I can see, but the smell fills the garage. I think I notice it when the air conditioner has been running, but really I have no direct connection to what may cause it. I never smell it inside of the car. I left the car outside of the garage last night to see if it was actually something IN the garage itself (like an open gas can), but there wasn't and there is no denying it that sometimes the car just smells like gas. Is this dangerous? Recent repairs: Front wheel Hub assembly. A mechanic also recently welded exhaust under car where pipe meets gasket then more pipe because the gasket had a leak. Now there is nothing but pipe under there. We had an oil change today and they looked for some cause, but found nothing, including no smell. But when I got home, it was back! Any ideas?