1998 Ford F150 loss of oil all at once

The truck does not get driven very much, since my husband died two years ago. My brother-in-law went to use it. Once he got home about 20min away and parked it. He called and told me that it had oil all over his driveway. The truck has never leak oil before.I looked at the truck.. all the oil was gone, no oil was on top of the engine, the oil was all under neathe. I did NOT notice the oil looking milky as in a blown head gasket. The drain plug and oil filter were in place also. Do you have any suggestions? I really would like some ideas before taking it to a mechanic. I am worried about taken advantage of.