If it's an old car and not doing more than a drop or two a day on your driveway, don't sweat it. Things happen like this as old man time wears away on gaskets, seals and other mechanical parts. If it's newer (fewer than 90,000 miles) it truly does depend on where on the engine you see the leak. Lower can be crankcase, upper can be value cover. If it's the head gasket, you'll know! Otherwise, find a local garage your neighbors recommend and get it checked out. Again, if it's a SLOW leak, don't sweat it. I ran a 1982 Toyota with a drip per week crankcase leak for a few years and never had a problem other than cleaning it off my driveway every month or two. I'm cheap that way.
A small leak in the vapor system from the tank to the engine.
transmission cooler
You might have a gas leak in your front engine compartment. The heater pull in air from the front and therefore if you got a gas leak it will pull the smell in get it check asap.
they leak oil from the oil pump on the front of the engine
because it may have a leak so yeah a small leak
An oil leak.
An old gasket that needs to be replaced.
Leak in exhaust system?
Bad seal - needs replacement
the purge valve on the gas line is rusted inside. It is located inside the left front wheel well behind the cover. Easy to replace. check youtube for video.
HEadgasket failure, internal oil cooler leak.
there are freeze plugs on side of engine if the leak is at the front, check to see if coolant is leaking from water pump and running down to bottom of engine