I have a strange thing happening and hope someone can explain.

I have 3 seperate push button circuits, each assembled as follows :

12 v + to a number of push N/O buttons ( like panic buttons ).
Return from the push buttons goes to a C106D1 gate ( right pin 3 )
The center pin (2) is connected to 12 v + permanently.
The left pin (1) is connected to a 1k resistor and flashing LED.

Of the 3 circuits, 2 of them are triggering the gate without the button being pushed. It seems to be random. No specific time interval ( usually a few hours or longer ).

The length of the cables from button to gate is about the same on all 3 circuits ( about 15 meters of 12 core solid strand cable ) and the 12 v + for the button is also supplied via the 12 core cable ( power supply is near the gate ).

Can anyone explain this or give me some ideas on how to correct it ?