I take my belt, watch, pocket change and wallet out in the line, I carry a microfiber cloth to wrap the watch in, if flying internally they all go in my briefcase, if on jollys then in hand luggage.
1 in 4 are taken for a pat down anyway so it could be awhile before you are reunited with your tray, so if its all hidden away in a bag then there's less chance of it getting swiped it's harder to conceal a bag and that would be an easy spot on CCTV. but somebody dipping in for just a watch/wallet may be harder to see.
Make sure you wrap your watch properly I have an elastic band which i wrap round the cloth to keep the watch safe, proved worth it in Italy as the security wanted my hand luggage through again and he purposely shook the bag and turned it upside down and plonked it back on the tray, my other watch was in it's watch case and it avoided some nasty dinks.