Just had the exact same problem with my IS last week. The fix is simple, the coolant level is slightly low in the radiator probably not by very much but it does need topping up. Mine did this because the expansion tank was dry so coolant could not be sucked back into the radiator as it cooled.
Simply remove the radiator cap (when cool !) and top the radiator up with coolant, replace cap, then check that coolant can be seen to be half way up the expansion tank by looking at the window in the back (check owners manual for the location of this). When hot the radiator can push coolant into the expansion tank so it needs to be able to suck it back in when it cools. Check the little thin hose from the side of the radiator cap to the bottom of the expansion tank as this can cause the same problem if it is split, perished or loose ! :winky: