Thanks for the replies guys. Actually right after I posted this question we went out and checked the pressure...there isn't ANY.
It looks like when about 6 months ago when I hit an empty oil case cardboard box in the road and didn't APPEAR to have done any damage that I actually did! That would also explain why I didn't notice it until now, since it just started getting warm out.
Now I just need to find out where the leak is...
