I go through security at Krakow very frequently. I know not to have any expensive items on me and absolutely nothing in the hand luggage. With watches and other expensive items, you could always put them in hand luggage beforehand as there is less likelihood of that going missing.
It does happen - rising problem especially in the US - there are thieves who will book cheap flights and see what they can pick up. I lost a very nice silver keyring this way at Budapest where the security line is quite chaotic. Unlikely to be security staff.
Put your phone, watch etc inside a bag or even just in your jacket and that makes it much more difficult for a thief who only has a couple of seconds to act
It's good advice.
Plus, if you've anything easily nickable in your checked luggage, bury it deeply; baggage handlers everywhere are notorious for opening bags and taking things out.