Sticky Keypad Protector Mystery

My 3 yr old safe, which is in a climate controlled room away from the sun, has a keypad protector that suddenly developed a strange stickiness. This is just the ring around the outside of the keypad, the keypad itself is unsullied.

At first I thought some type of liquid leaked onto it but it is even throughout. I am pretty sure this only developed in the last 2 weeks or so. I was out of town for a few days in that time but no one had access to the safe.

Pictures attached. Any suggestions on what could have caused this would be much appreciated. Thank you!