@Reine Oops, you're quite right. M106 S255 (which is probably what the slicer is producing in the Gcode) doesn't work, but M106 P0 S255 does. I thought M106 without a P parameter defaults P to zero. But I think it must default to the tool fan, which makes sense in a multi-tool setup, because then you're changing the fan speed of the current tool. But if no fan is associated with the tool, it doesn't fall back to F0. I've always just had no fan specified in M563, so it DOES default that to F0. I put F-1 in yesterday, but didn't run a print to realise it wouldn't be controlled by slicer Gcode.
I think you may just have to live with the slight annoyance of seeing 'Tool fan'. Sorry!
Ian