A hair dryer or hot air gun is the way forward, be carefull not to get them to hot or they will stretch and break, then youll have to pull off loads of little bits!
Any adhestive left can be removed with a small amount of meths.
i was told petrol would remove them, also call at any of the car part shops i had some selotape i couldnt remove and the guy sprayed clutch fluid or something else in a can and removed it with one wipe, but dont quote me on the clutch fluid its something used for cars in a tin ask the people in the shop it must be a common tip because this guy wasnt keeping it a secret. PS if you try the petrol DONT SMOKE
It was probably WD40 rather than brake/clutch fluid. Petrol will remove the residue followed by T cut or rubbing compound and then a good wax polish otherwise the marks will keep coming back.
Unfortunately it is very rare not to be able to see the marks unless the vehicle is quite new.
It would more than likely be an aerosol brake and clutch cleaner that was sprayed. we use it to clean sticky residue off almost everything. does tend to fade the paint a wee bit. t-cut brings it up a treat.