In the event of the drone malfunctioning it is programmed to fly back to base, it is highly unlikely that this drone which can nearly ten miles up would be in such good condition if it was shot down, indeed it looks like it was hijacked electronically from the ground and landed in Iran. Do not underestimate what technology Iran may have as only a few months ago North korea managed to force a US drone from its airspace using sophisticated jammers and both north korea and Iran are supposed to co operate according to US intelligence. it looks like the iranians may well be telling the truth!
Saudi-backed forces advance on rebel-held province in Yemen (DPA)
from the article: Iran says it will not return captured U.S. drone