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@Greenbank
They use an acid to etch a number into the frame on the underside of the downtube. The numbers are then a very visible blue or black. They also give you a big red sticker to go on the top tube (or somewhere visible) so that thieves know it is marked.
And, yes, there are different acid/etch types for different frame materials, I've had 3 frames marked. One steel, one Alu and one carbon.
The idea is that etch goes deep enough that it would be obvious if someone had tried to sand/grind it out.
Personally I wouldn't mark my Colnago (90's Master with Art Deco paintwork) but that's because it's got the Gilco pinched tubing. (Plus I'm looking to sell it.)