red flashing sensor on the dash above water temp needle?
if so then it sounds like the coolant level sensor. is the needle sitting at 90? if it is and everything seems fine then defo the coolant level sensor.
If the coolant levels are down then top up and go for a run preferably from cold and see if it still comes on. If it does and the the car is under warranty then have the dealer fit a new coolant bottle -
problem solved ... for a while anyway until the part fails again.
if out of warranty then open the coolant bottle lid and you'll see two silver long pins that stick into the coolant. scrape these where they meet the coolant with a screwdriver to get off any deposits and try again. worked for me. saves you about £15-30 for a new bottle/sensor.
they are prone to fail. on my 3rd in two years.