How could they be caught? I wouldn't know how anyone could detect it.
some exchanges are quite stupid when placing orders thereself.
I already detected some, most do not exist anymore, so it really are mostly small exchanges with a few users.
I detected it by finding a pattern in placing orders. One for example always placed big buy orders (several > 30 BTC) always 5€ below the best user order.
And since they did not build in a limit and there were only very few user orders, I placed a buy order at 570€/BTC (while it was 500€ at other exchanges), let the exchange place their big orders at 565€/BTC, quickly removed my orders and placed a sell order for 565€/BTc which was filled.
I did this quite often and made good profit. But of course the exchange refused to pay out the money/btc I made , reverted everything back and build in a limit.
At bigger exchanges it might be much harder to detect such activity.