The chances of it being a bomb are that low its, stupid. That said if there was a chance that it was, what would be the best course of action. Keep in mind there have been instances of terrorists remotely activating them before, in fact if I remember rightly one bloke had one up his coight.
I would imagine that a remote activation of a device on a fully laden aircraft clambering into the air over a residential area would be one of the worst places for it to happen. Hence would have thought the if you really thought it was a bomb your first duty was to open a window or door and throw the thing as far away from the aircraft as possible, including the wing. I would imagine you would do this first before notifing ATC.
Why? you don't want any one watching or listening to notice something wrong and just let it go anyway. But as I stated at the begining, it would be the last thing on my mind, and maybe that's just a lack of imagination on my part.