Engine light

The oil and filter changed at same shop every 3,000-3,500 miles. Vehicle has 136,000+ miles, fair amount of highway miles. All other maintenance kept up on veh. While driving low oil light came on twice, stopped, almost immediately close to service station. Waiting awhile checked dip stick barely any oil was drop reading on stick, 4 qts of oil. Filled resevoir ok. Parked car went in am to shop they said-we looked no evidence of leaking under vehicle-engine compartment dry. They let it run for awhile no evidence of smoke showing it was burning it? The oil is clear, very clean, not milky yuky. No oil puddle where vehicle is parked on regular basis. Its been 5 days since this & dip stick is showing about 1/4 low? I was told there is no "oil sensor" that would have warned us it was getting low. What is happening? Any one report this type of thing to their toyota? We love the car and are concerned about engine damage? Going to carry oil; but use the veh for a fair amt. Of highway driving.