I went up to see my parents over the weekend (260 mile round trip) and took the Polo for a bit of a blast while she disappeared off on a hen weekend in my car

. When I came back yesterday I noticed that the coolant reading on the dash went back to zero a couple of times (only in the first half an hour hen it was fine for the next two hours).
Does this mean the coolant sensor is on it's way out? Is it a common thing? Anything else I should check first?
Thanks in advance