I would suspect that most airport screening for radiation looks for Gamma rays- radioactive rays that can pass through many layers of objects including steel and lead.
Polonium 210 emits Alpha rays which are blocked by almost anything.
Litvinenko ingested Polonium 210. Initial screening for radiation poisoning came back negative despite symptoms that were in line with radiation poisoning.
Polonium 210 dust is likely to have been spread by Litvenenko and others, but its pretty much harmless unless ingested.
There was a very good Wall St. Journal editorial earlier this week explaining it all

I do wonder if it’s the old ‘diplomatic pouch’ trick, especially since he did level accusations at Putin before he died. Yes, it did make the news over here, heck even Rooney’s new contract made the DC news. The world is getting smaller…