How are the lenses held in? Got home tonight and one of my lenses is missing

They are the original lights as well
Does'nt look like it's been hit with a stone cos there is no sharp bits or anything it has just gone. Played with the other one to see if they twist out and that one fell out in my hand so i'm lucky i have'nt lost both. They look to me that they are held in with silicone or something. I'm putting it down to the age of them and the vibration has caused it to drop out. Don't think anyone has pinched it as there seems to be no sign of it being prized out.
Don't suppose anyone has one do they ?
Numbers on it are . . . carello 640 HR E3 17.5 38628 06 822.700
cheeRS