I'm on my second character to run the campaign now, so have an idea what to expect.
Got to the first of the Expedition Quests, "What Lies Beneath" and dutifully completed the first section of the expedition with no trouble.
Then when I got to the second section, I reached a chest and opened it. To no one's surprise it was a bloody Mimic. The only problem was, it was a level 80 Mimic, and utterly relentless. Even with an ITC'd Stealthed speedy Rogue, I couldn't get away from the little bugger, and was aggro'd so unable to open the door. I had no hope of killing it, (believe me... I tried...) so had to die to escape it. Thankfully it reset.
All the other beasties were level appropriate, but it might need someone to scan over the inhabitants of the Intro expeditions, to see if it's a one off, or a permanent resident.
What I would say is... those chests are pretty crappy anyway... so I'm not opening any more in the intro Expeditions. They aren't worth the risk of a run in with Invinci-Mimic.