What kind of sensor did I see today?

["SecurityNovice" <nospam@xxxxxxx>]

Hi all.. I am new here.. Looking to learn...I think the more I think I know,
the more I realize I know so little...  I came upon a very interesting
install today.. or whats left of it.  Place is in an old building undergoing
extensive renovations.  There is an old door that resembles something out of
a stable.. cobblestone drieway, etc.. has to be 75 years old.  Some old time
alarm guy looks like he protected this door by using what appears to be very
small guage wire in a back and forth criss cross patern on the iside of the
door.  I imagine if someone kicked the door in, or maybe kicked at it from
the outside, the line would stretch, or maybe break?? (not sure on this
part) and cause an open or closed condition.  Has anyone else seen anything
like this?  I love goingi into old gun shops or jewelry shops and seeing how
the old timers did the foil on windows..   Even new systems.. I enjoy
walking into a store and looking around.. wondering why they opted to go a
wall mount motion when a 360 ceiling mount would have been better... then
trying to figure out the logic behind the decision..