The mystery of the missing oil?

So typically the dipstick is not at he bottom of the oil pan. That may not mean you're out of oil it may just mean your below the bottom of the dipstick. You could drain the oil out see how much is left or go ahead and add oil until it shows up on the dipstick again. If this is the case then it's not unusual to burn one or two quarts in an older motor over a period of several months depending on run time.

You didn't mention how much oil you had to add!