The main problem is that your Raspberry Pi looks exactly like the Hollywood bomb controller in the movies (circuit board with strange wires). The folks on the door can't see that the Raspberry Jam is all about those small computing things with funky wires sticking out so they make the wrong assumptions.
The blame lies firmly with the organisers who should have run a better education session for the security folks. Tell them what to expect, tell them how to decide that a "device" is someone's experiment that they're going to demonstrate and have the tools (tv, keyboard, mouse) available to plug in and test the things they really don't understand. Have an organiser there as an expert to consult on the things that don't make sense to the security staff. Have a register of the folks who are expected to bring their experiments (they can be told to carry a photo driving licence or passport) so everyone can play nicely.
It really isn't difficult and it really shouldn't be left to the poor folks who've paid for a ticket or the poor, underpaid, badly briefed security folks who've never seen this stuff in the field.
We need to be vigilant, but we don't need to be stupid and over controlling.
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