That said, why wouldn't a Mimic cough up (!!) some loot once it was bashed into deader than dead?!?
On the issue of Spotting, yes, it was interesting when 2/3 of the PCs in our group spotted the Mimic. Of course, if you Spot it, you can't do anything—it just sits there. But when our 3rd member couldn't make the Spot check, it "woke" the Mimic, and we had to battle it. I guess that mechanic is a bit strange, since if you recognize the Mimic, you can't choose to fight it, which I would say your PC should be able to choose. Thus, the only PC that will have a hassle with a Mimic is one that can't spot it.
DESTROY ALL MIMICS!!!
But on the run away issue...you can't really design for run away when Death has no real meaning. No sensible Player is our adventuring without Raise Dead scrolls, so in a group, you'll almost always survive, unless you're being absolutely OOC stupid in your play. And even if one is soloing with their PC, you no longer earn more XP/loot than with a group, and if you do perish in your solo exploits because choosing not to run away, the player is far more likely to just spawn and return to a) get the rest of the Areas loot; b) to regain that lost XP.
It is only when a mechanic to "disable" or "cripple" the PC so that adventuring and xp/loot gain is rather impossible for X time, will it create absolute incentive to choose run away—and then return...—over just plowing through the content and taking the risk in a much more OOC manner. Imho.
Again, don't get me wrong: Mimics are awesome! Atm, mechanically, they are a challenge. Shouldn't that challenge (ie risk) also earn a reward?