@ skirdlov
The chances for any malicious device to have been planted at any previous stopover decreases exponentially as one goes back (Paris being the most likely). Paris, Cairo & Tunis all have their fair share of loonies, in this respect I would not single out any. Also not mentioned here previously, the role of those three security personnel on board is not limited to in-flight, they also maintain ramp security during turn-arounds, and they are the ones who actually open/close the cargo doors. Anyone trying to gain access to the avionics bay would have to dodge them. Not saying it cannot be done, but rather unlikely during a short turnaround.
For those suspecting the catering trolleys, they undergo a security screening prior to loading (lust like checked luggage) at most airports I'm aware of, Cairo certainly included. Again, things have been missed by screeners before, but still...
Last, Egyptair is not an obvious target. Inside Egypt it is a National institution, and while there are several groups opposed to the government, their main targets are the armed forces or tourism infrastructure. It could be expected that an Egyptair flight would mainly be full with Egyptians, not foreigners (as was the case), it would be a very atypical target for any local group. For foreign groups, targeting a western carrier with the same effort creates much more impact. The terrorism theory does not really add up, especially as there have been no claims in the past 3 days. One may always argue about the lone perpetrator, but getting a device on board an aircraft these days requires considerable skill and organization beyond the means of a single untrained fanatic.