Originally Posted by Sweetone
I doubt you know me well enough to guess. The only reason I haven't responded sooner is I'm trying to think of some.
Let's see - It would probably take a combination of things to really get me nervous.
A passenger putting something in the galley or lavatory - of course depends on what it is.
Sorry, but I have to

at the thought of that one.
An individual or several people trying to distract or block a FA.
Most of the FAs that I've seen in the past few years are pretty vigilant, and
would probably be far more aware of risk factors than even the most seasoned travellers. I'd help if I saw signs of distress, though.
A glimpse of some type of prohibited weapon - a real one, not the bogus liquid type.
Too iffy. I prosecute weapons offences on a regular basis, and I find it extremely difficult to differentiate replicas from the real thing without careful analysis, something that is just not possible with an eye......g. One of the exhibits at one of my trials yesterday was something I first thought to be a small handgun. When I yelled at an attending officer to keep the damned thing secure, he smiled pityingly and told me that it was a cigarette lighter.
Sudden changes in behavior - such as a group seen together in other areas of airport who suddenly act as complete strangers
Oy. Have you ever travelled with family members?
Adults who pay too much attention to children traveling alone. (I'm a mom)
I'm not sure that I'd want to assign negative connotations to people who may just be looking out for stray youngsters. If we yell "predator" every time a good samaritan's acts are misconstrued, they'll stop acting, and THEN something bad will happen.
These are some examples of behaviour I would continue to observe. Of course they could all have a logical explanation, just something to keep an eye on.
Ned69 - You gave me a good laugh.
Seriously, I'm not trying to pick on your suggestions. It's always wise to be vigilant, but I'm well aware of the limitations of my own observations as well, and try to rationalize odd behaviour first before acting on my own impulses.