Gareth, (and I am feeling like I've slapped a puppy here ) as much as I'd like to think you randomly lost TWO caps simultaneously (the caps have retaining spring clips which would adjust for heat/fit - so could only really come off if they weren't put on correctly), I'd guess that you were, unfortunately, struck by a vulture-tea-leaf.. 'stards that they are..
Glad to see replacements aren't too expensive though