I would have trapped the beetle as you did. Since you could not leave work right away to go back to the shop, I would have called them to let them know that I will be stopping by with the said beetle with their salad after I get off work. Hopefully you still have your receipt and the markings on the container identify the store so they can't say..
"Oh that is not our salad".
The outcome depends on how you handle yourself. If you go in raging mad and confrontational, you won't get as far as being sweet.
I would expect a replacement salad and if management is on top of their game, maybe a gift card for another meal as well.
Let us know how they react and what they do to remedy this.
Good luck