Outside possibility to check here:
When I bought my SAAB 9-5 the ABS light came on while I was on the way home, I took it to SAAB for a service the following week and while I was there I asked them about the ABS light, they checked the fault code for nowt and told me it needed a new wheel sensor.
Had a look about and hummed and hawed for a while then it hit me, the tyres on the front had a lower profile to the ones on the rear and one of the front tyres was a little flat looking, I pumped them up and thought no more of it.
When I went to get a new front tyre after a puncture they fitted a tyre without checking what was on it and put on a different size tyre.
On both occasions (when I got it-flat tyre of wrong profile and when I had a tyre changed) the ABS light came on after a few miles as the system was noticing the wheels were turning at different speeds.
The fault code came up with "incorrect number of teeth" for one of the sensors, after the tyres were sorted it never came on again.
Just thought you should check it has the same profile of tyres on all wheels and that all are properly inflated as both can cause the ABS light to come on and may even record a fault that makes it seem as though a sensor is faulty, costing a fair bit of dosh at the garage.