All baggage irrespective of origin is xrayed using the latest in technology and the bar code number is scanned into a system known as BRS. If it's not in the BRS, it's not on your aircraft. If it's on your aircraft and not on the BRS, a problem is there. But how do we know, we have to trust our ground staff implicitly. (that goes down to security screening - but thats another story)
Cargo is either screened by xray if small enough or held pending a detection procedure or a prescirbed time limit prior to loading.
The DTlr here in the UK, have very stringent regulations on un-checked and unaccompanied baggage on aircraft. If they believe and operator knowingly allowed such items on board an aircraft and failed to resolve the situation immediately, they do have the power to impound aircraft, levy very hefty fines and finally suspend an operators AOC licence for UK airspace. Draconian I admit but why risk even the slightest wrath of the authorities even if you can be sure that the bag is safe and has been xrayed.
Having just completed an audit with an inspector, I can tell you human they are in appearance but god like in wrath.