"Hybrid cars are targeted by thieves because they have two power sources – an electric motor and a petrol or diesel engine – so their catalytic converters are used less frequently to process pollutants. This means the metals inside them are less likely to corrode, so they're more valuable and therefore more desirable to thieves."
What you learnt in Chemistry might well be correct but the quote above, which can be found everywhere on the internet, echoes what every mechanic I have spoken to, what the police told me when my RX450h cat was stolen (in 30 seconds) and what Lexus Reading told me when they sold me a new one at cost (half price). More crucially, it is what the thieves believe and that is why hybrids are being targeted more often than non hybrids. If your Chemistry teacher was right, and hybrid cats are not cleaner then I wish the experts out there would spread the word and let the thieves know. Then we can share the joys of this crime wave on equal terms with drivers of ordinary petrol cars!