The lithography machine itself does not create the circuit. The circuit master itself is encoded on a so called Reticle. This reticle (sort of like a transparency slide) is a (usually 4 times) enlarged master of the circuit, which is then projected onto a resist coated wafer. The resist hardens or softens (depending on type) on the exposed areas and is then washed away, after which etching takes place.
The production of these masters or reticles is not done by ASML machines but by specific machines designed to do this sort of thing. I know for a fact customers inspect new machines very closely. A piggybacked circuit burned into the reticle stage clamp (the only way I can think of that MIGHT work) would only work for a very specific design of chip where the designer was even kind enough to include all vias, and take the timing and capacities of all this extra circuitry into the design of the rest of the chip. On top of that, I highly doubt a machine with a hardcoded reticle would get past any acceptance inspection by the customer. SOMEONE will notice. On top of that, most chip fabs run very extensive pilot batches which get 100% inspection. Any aditional circuitry that should not be there would get flagged. It would be seriously much easier to hack the chip fabricator themselves and slip the design into the chip itself. It's just as likely to get noticed.
Check out this explanation of the lithography process: http://www.lithoguru.com/scientist/lithobasics.html
Then keep in mind that only the "alignment and exposure" step is done by the ASML machine. After that they might come into view on SOME customer sites with its Yieldstart inspection tool on the metrology steps.
That visiting technician might simply have seen a new chip design or a test design that he wasn't supposed to see. IP protection in production fabs is VERY strict. Even visits from service techs and the likes are planned AROUND the times proto chips will be cycled to avoid any chance of someone seeing something they shouldn't.
(disclosure, I work in this particular industry and have visited the mentioned company on many occasions)