While being in Philadelphia I thought I had to do some historical touristing things. So I visited the Liberty Bell. Most interesting detail was this electrical installation. After talking with the security people, I found out this is not a burglar alarm or such, but instead a measurement instrument they installed when they last time moved the bell. After placing it where it is now, they decided to leave it. The idea is that the instrument is to warn if the crack widens or if something else happens with the bell that in fact is made out of pretty crappy metal.
But I found the one in Lego at the airport also be interesting.