I seem to recall that when a similar situation was recently discused on here it was mentioned that with some a/c it isn't all that easy to see up into the compartment (begs the question of how someone gets in there, I know!!) so a quick visual-check by the ground-handlers mightn't spot anything!! Comments were made (and quickly dismissed) about putting cameras in the wells linked to the flight-deck but I guess it really comes down to security at airfields particularly when at hold points prior to take-off