Andy in theory you can still remove the DPF without any signs. However it's still an offence. The government could go after garages (unlikely) but what most likely is that customers who have failed MOT with DPF removed would be very p*** off and that could result in few garages splashing on replacing DPFs because legally they are responsible.
"Technically, it isn't an absolute offence. The offemissions s to use a vehicle which has been modified in such a way that it no longer complies with the air pollutant emissions standards it was designed to meet, not to remove the DPF" - technically they could argue that you were defrauding them by not paying the correct amount of road tax, which, correct me if I'm wrong, is at least partially based on the amount of air pollution
A visual check is never going to be good enough, DPF cars should be emissions tested to avoid this issue.