Coolant light but level fine

[gumby6371]

As it says in the heading my coolant light comes on quite regularly for a few seconds then goes out, the level is fine when I check it, temp gauge never goes high and fans kick in as normal at 90.
Seems to happen mainly under acceleration in 3rd and 4th gear.
The only thing I can see is what looks like a bit of lime scale round the seal in the coolant reservoir so probably needs a flush but not enough to do any damage.
Any thoughts? presumably sensor related?

[fuvureju]

faulty coolant level sensor?