Can bad guys really hide "dirty bomb" material via bananas? This one probably has some cross-over with Skeptics SE, but I thought it fits here a little better.
The concept comes from the TV show NCIS:LA second season episode "Empty Quiver"
Here, the villains steal some nuclear material from a hospital's radiology department (in itself dubious).  Then in order to transport it across the country without detection, they surround it in a truck full of bananas, as if it's nothing more than a produce delivery to a grocery store.
Ok, so let's accept that bananas contain potassium, that there exists an isotope of potassium that is radioactive and found normally in nature (including bananas) and other things as indicated here.
Is this collectively, i.e. an entire truck load, enough to fool a trained inspector?
 A: This doesn't seem likely for several reasons.  Most hospital waste is considered low level waste (LLW) and not very hazardous and would make for a anticlimactic dirty bomb.  In fact LLW is usually just incinerated or buried with only minimum precaution.  Let's assume they got a hold of something that was not LLW waste and was a high level waste (HLW) item, like in Goiania.  
Something that would actually work in dirty bomb would have to be stored in a shielded container, thus you wouldn't need bananas anyway as bananas radiate very little in comparison to almost everything else (see below).  But if the crooks were as ignorant about radiation as the scrap metal recyclers in Goiania who spread the glowing powder and dust around and even let a kid rub it on her skin for effect, then perhaps they took it out of the container to transport it.
At this point the exposure levels would be so high that bananas wouldn't even compare.  So other than providing a snack in between bouts of vomiting from radiation sickness I don't see what bananas would do to mask it.  Banana radiation does make a nice little plot trivia that helps the story seem more realistic.
Let's compare some numbers in banana terms (BED banana equivalent dose):

80 million BED = Fatal dose even with treatment
20 million BED = Severe radiation poisoning, fatal in some cases
500,000 BED = Maximum legal yearly dose for a US radiation worker
40,000 BED = Ten years of normal background dose, 85% of which is from
  natural sources
4000 BED = 1 Mammogram
1000 BED = Approximate total dose received at Fukushima Town Hall in
  two weeks following accident
400 BED = 1 Flight from London to New York
300 BED = Yearly release target for a nuclear power plant
200 BED = Chest X-ray
50 BED = Dental X-ray
1 BED = Eating a banana ( 0.1 μSv = 0.0001 mSv dosage)

That's a lot of bananas to get close to a dosage that would be fatal, so it would take a lot to cover up an unshielded radiation source.  In fact our bodies carry around more BED doses than a banana.  
We surrounded by naturally-radioactive materials, and are constantly bathed in radiation originating in the rocks and soil, building materials, the sky (space), food and one another. A typical background level of exposure is 2-3 millisieverts per year (mSv/y) or 20000 to 30000 BED / year.
Bananas probably wouldn't provide a lot of cover.  After all don't they scan a lot of organic products and know how much to expect?  So anything additional would be suspect and HLW would stand out unless it was so well shielded that there would be no point for bananas anyway.
A: It's not entirely impossible, but very unlikely IMO. There's not enough information to know.
First, what type of inspection were they were trying to evade? For example, if someone drives a small truck from the east coast of the USA to the west coast, they normally would not be inspected at all. For those who have not been to the USA: There are not routine inspection of vehicles at USA state borders, like there are at many national borders. You just drive right through from one state to the next, all the way across the country.
OK, let's assume that for some reason somebody is checking the radioactivity of all passing trucks. (Maybe the FBI has been tipped that a dirty bomb will be on a certain highway?) There are ridiculously sophistocated nuclei-detection systems out there that would have no problem finding a weird isotope in a truck of bananas. But maybe the FBI doesn't have such fancy equipment, or can't transport it to the inspection stop very fast, so you just have inspectors holding off-the-shelf geiger counters near and inside the truck. I guess it could happen, why not?
Different radioactive isotopes decay in different ways (e.g. alpha, beta, gamma radiation). We don't know what they stole from the hospital. If it was an alpha or beta emitter, the radiation can be blocked by a thin shield, like a suitcase, so the bananas would be pointless. Maybe they have a gamma emitter? But bananas are beta emitters, so the sides of the truck would block the banana radiation but not the gamma. If the inspector saw a signal outside the truck of bananas, it would prove that there was something besides bananas inside the truck. On the other hand, maybe the inspectors are not very well trained! They could reach the wrong conclusion.
So if there was an inspection, and the inspectors had bad equipment and bad training, the bananas could be helpful in confusing the inspectors. But in most plausible circumstances, filling the truck with anything else would be equally effective as filling it with bananas. (That said, there's nothing wrong with bananas, and you can grab one for a nice snack during the long journey!)
