I have a 2001 Nissan Sentra and recently had an oil change last month and a coolant flush just last year. A few days ago, I noticed the needle of the temperature gauge went up to the 3/4 mark while I was driving on the highway. It has never happened before and only happened once. A day later, I drove around several blocks to run some errands and it remained at the normal 1/2 mark. Should I bring it in for service or keep monitoring it? I checked that there is coolant (full) in the overflow tank.
Unless its a particularly hot day, when driving on the motorway the coolant temp should be around whatever the normal area is for your vehicle, as air is being forced through the radiator at a great rate at motorway speeds.
If this was just after starting the vehicle from cold AND it was a warmer than usual day, then perhaps the coolant just got a bit hotter than normal before the thermostat opened, however keep your eye on it. If it happens again have it investigated.
I know you said the coolant level in the expansion tank was ok.. But check the radiator too as it may be low on coolant.. Sometimes the expansion tank shows full as feeder pipe can be blocked or squashed, preventing coolant getting back into the radiator when its required.