Engine Management System

[Jodaw]

I have a 2005 Yaris - my engine management warning light came on (malfunction indicator lamp). At the same time the low engine coolant temperature indicator light in blue came on. I checked coolant levels and that was low so topped this up (only water was available). The following day the vehicle started ok and all warning lights were off. I drove it for 2 miles (there is no loss of performance) but after about 2 miles the malfunction indicator lamp came on again together with the blue coolant light. A check of the coolant shows that it has not lost any fluid. The car was serviced about 4 weeks ago and I am wondering whether they failed to fill up with coolant. My question is whilst I am booked in to have the car checked out is the probable cause related to the lack of coolant?