Re: the grenade, I agree that these cases need to be taken seriously and that evacuations are sometimes, maybe even often, warranted.

But I do think there are cases where TSA could ask the passenger/owner a few questions and resolve the alarm without calling in the bomb squad an evacuation.

In a lot of cases, simply asking the passenger a question like "do you have anything that might resemble a grenade" and then evaluating the response alongside the totality of the situation (e.g., 85-year-old passenger, sheepishly admits he forgot his souvenir, etc.) would help. If TSA trusts their vaunted "behavior detection officers," (BDOs) then they should use the BDOs to evaluate responses in these cases. If the BDOs are worth their cost, they should be able to tell the difference between a sheepish souvenir carrier and a nervous/malicious terrorist in the vast majority of cases.

There have been too many cases where a power supply wrapped in some cables, or a slightly unusual electronic device, have resulted in TSA freakouts that probably could have been avoided with a simple conversation.
See http://www.flyertalk.com/forum/showthread.php?t=534358 for an example and http://www.flyertalk.com/forum/showthread.php?t=515053 for another.

Some of these incidents have resulted in serious property loss for the passenger with no compensation from TSA or other agencies. See:

http://www.flyertalk.com/forum/showthread.php?t=368482
and
http://www.flyertalk.com/forum/showthread.php?p=4991865