I don't recognise it, but it's not that unusual to find unwelcome additions to bagged salads, and I've usually only found them when I've had occasion to wash the contents prior to consumption, making me wonder what I've inadvertently consumed in pre washed, bagged salads...
The trouble is, weeds will grow in with other crops, often unnoticed by the grower when on an industrial scale. I've found it's more likely for random leaves to appear in organically grown salads, though I've never found any quite so large as your sample in the pics. I'd be inclined to send or show the photos to the retailer, or, better yet, the producer, seems their quality control needs a bit of tightening at the very least.
It might be Hyptis suaveolens, that has short bristles on the undersides of the leaves - it won't kill you, but its more usual to use as a tea for diarrhoea! Both Rocket and Hyptis like similar growing conditions, but it's hard to be certain what it is because it's only part of a leaf.